No audio in on an M1 Mac mini

You can split the headphone output of the mini using a TRRS splitter like this one https://www.amazon.com/gp/product/B01I3A47I4/ref=ox_sc_act_title_1?smid=A30K982V62WUOW&psc=1, then connect the 3.5mm audio to it.

The problem is that Mic In on the mini and Line In which your Amp wants to feed to are at different levels. I would bypass both the amp and the preamp first, connect direct from player to the splitter. Then connect them in sequence checking for the best recording quality. Good luck!

Where from and to what (program)?

What record player? Does it have line out, or go to an amp with line out? Details, please.

Any audio input using the Mac mini's headphone jack is going to be monophonic. If you wan't stereo, look for a USB audio interface,

It depends on the record player and also records. Arguably, true audiophiles will prefer mono as it’s how the LP artist(s) intended it to sound (stereo LPs were post-processed for signal separation).