'Connected to the internet is a bit vague'. If you're using a standard router nothing'll be able to access the XP machine from the internet to begin with due to NAT, unless you manually forward ports.
If you just want to prevent anything on XP from accessing the internet, you could manually configure its IP address settings and not configure a gateway. Normally, it'll get this automatically over DHCP and your internet router will set itself as the gateway.
Another approach would be to keep DHCP enabled for the XP machine, and to add a firewall rule in your router to prevent it from actually accessing the outside world. And disable UPNP so the XP machine can't ask for ports to be forwarded.

Ok... Now we come to the land where I don't understand that much... 🥴😬 But I will read to get it ...
As long as I don't know how, I will go on using my usb drive..

"Connect to Internet" is very antiquated term. It was meant back when people used dial up modems, or connected their computers directly to a external modem. Thus you had no firewall, no Network address translation via routers, no VPN, no nothing. Connected to Internet means if I am in VA and you are in CA and this for example is your IP and I type Ping 13.23.16.40, than your computer literally responds to me. Theres no such thing in 2021 as really "connected to internet" Maybe a Phone counts as that.

you would have to explain to me more about how your network is setup and what your expectations are, I mean more specifially what it is that you exactly want to be able to do.

Without that info all I can really say is these days "connect to internet" technically means does your computer have a "defualt gateway configured" which is generally the internal IP of the router and can that computer with the defualt gateway configured be reached VIA the "de-militarized zone" AKA "WAN" AKA Internet. If neither choice is true that its not "connected to the internet"

If you have the XP and Windows 10 machines on the same network and you want to connect to the SMB share on the Windows 10 machine then you'll need to enable SMB1 on the Windows 10 machine (vulnerability), change some security settings in gpedit/registry and make sure you have a user account created for the XP user and that user has permissions to the share and file system onthe Windows 10 box. I just use FreeNAS,much simpler.

This is pretty easy, It looks harder then it really is.

Step 1. make sure both machines are in the same workgroup, name doesn't matter just as long as they match.
https://www.linksys.com/us/support-article?ar … leNum=140590#a1

Step 2. Log onto both PC's using the same username and password, You do want a password even if it is just 1 character.
You don't have to do this but by default windows will use the logged in password to access the other PC. if these match then your in right away. If not you get prompted for login details.
Vista and later get a bit funny with blank passwords.

Step 3. Enable SMBv1 on Windows 10 PC
https://www.tachytelic.net/2019/09/windows-10-enable-smb1

Step 4. Right click a folder and select Share, the permissions are self explanatory .

That's all the steps that are required to have file sharing enabled between the 2 PC's. Windows can take a while to find other PC's on the network. Quickest method in this case is simply enter the IP and share name in the explorer address bar or start run eg
\\192.168.1.1\Shared

Personally I just let DHCP do everything and don't worry about the "no connection to the internet" The router blocks alot of traffic as does Windows inbuilt firewall. It's not perfect but a retro PC that spends more time off then on I don't see as a big risk but if you really are concerned setting a static IP is the easiest option.
https://portforward.com/networking/static-xp.htm

IP address: check your router and see what it's DHCP table is usually its something like 192.168.1.1 - 192.168.1.200.
The 192.168.1 (or whatever yours is) you want to keep the same, that final number is that PC's individual number. You want something that falls outside the table but not the routers address, not 0 or not 256 or above.
You can do what I do and set the DHCP table to only hand out IP addresses 100 - 200 which is still a lot for a home network and easy to remember what not to use when assigning static IP's
Anything currently on your network will auto update usually in the space of a few days without he user noticing a thing.

Subnet Mask: is exactly the same as the other PC's

Default Gateway: Leave this blank this really means Gateway to the internet

DNS: you can set this to the router or leave blank, In theory this would convert your IP addresses into computer names so you could enter "\\XPPC\shared" into the address bar rather then the IP, reality is you'll get used to using the IP address after a bit so it's not really needed

Yeah the SMB1 thing, to be more secure I've only enabled the client on my Win10 machine and, as such, copy files to a share on the retro (Win98 in my case) machine.

Some people are deeply paranoid about enabling SMB1 on Windows 10.

Apparently mucommander is an alternative, but I'm not sure how capable it is. (For instance, I'm not sure if you could run mucommander on Windows 10 and then connect to it with standard Windows networking in Windows XP.)

chinny22 wrote on 2021-03-03, 16:35:

This is pretty easy, It looks harder then it really is. […]
Show full quote

This is pretty easy, It looks harder then it really is.

Step 1. make sure both machines are in the same workgroup, name doesn't matter just as long as they match.
https://www.linksys.com/us/support-article?ar … leNum=140590#a1

Step 2. Log onto both PC's using the same username and password, You do want a password even if it is just 1 character.
You don't have to do this but by default windows will use the logged in password to access the other PC. if these match then your in right away. If not you get prompted for login details.
Vista and later get a bit funny with blank passwords.

Step 3. Enable SMBv1 on Windows 10 PC
https://www.tachytelic.net/2019/09/windows-10-enable-smb1

Step 4. Right click a folder and select Share, the permissions are self explanatory .

That's all the steps that are required to have file sharing enabled between the 2 PC's. Windows can take a while to find other PC's on the network. Quickest method in this case is simply enter the IP and share name in the explorer address bar or start run eg
\\192.168.1.1\Shared

Personally I just let DHCP do everything and don't worry about the "no connection to the internet" The router blocks alot of traffic as does Windows inbuilt firewall. It's not perfect but a retro PC that spends more time off then on I don't see as a big risk but if you really are concerned setting a static IP is the easiest option.
https://portforward.com/networking/static-xp.htm

IP address: check your router and see what it's DHCP table is usually its something like 192.168.1.1 - 192.168.1.200.
The 192.168.1 (or whatever yours is) you want to keep the same, that final number is that PC's individual number. You want something that falls outside the table but not the routers address, not 0 or not 256 or above.
You can do what I do and set the DHCP table to only hand out IP addresses 100 - 200 which is still a lot for a home network and easy to remember what not to use when assigning static IP's
Anything currently on your network will auto update usually in the space of a few days without he user noticing a thing.

Subnet Mask: is exactly the same as the other PC's

Default Gateway: Leave this blank this really means Gateway to the internet

DNS: you can set this to the router or leave blank, In theory this would convert your IP addresses into computer names so you could enter "\\XPPC\shared" into the address bar rather then the IP, reality is you'll get used to using the IP address after a bit so it's not really needed

This was a real substantial help. Thanks.
Almost forgot:... 😀 It works