All that it is just a simple socket 1 for what Intel used for their renamed 486 DX (487), it was a big swindle when people had only the SX model and people would buy the higher priced DX model as an upgrade thinking it would make a noticeable improvement. The only time that the extra socket is a good thing in examples like your system when the stock cpu is integrated. When you plug a normal DX or a faster clocked SX model 486 in that socket and set the jumper if there is one the original cpu gets disabled.

Yes, there's a jumper next to the socket, that disables the built-in CPU. I've heard that it's dangerous for 3 volt CPUs to insert into that socket because it would serve 5 volt only. What do you think?

Get a adapter that allows for 3.3v and all is set.

there are markings on the motherboard. Look for the corner with an A and 1. That is pin 1. Make sure any cpu you put in the socket, look for a dot and match the dot to the A-1 corner. Anything else will kill the board & cpu.