iPhone 12 buttons replacement guide

Unfortunately, there aren't many guides out there for button replacements on iPhones. The closest I can come is to point you to the guides for the iPhone X; fortunately, Apple hasn't changed the basic design of the buttons throughout most of its various models, so the principles will be the same.

iPhone X Charging Pad / Volume Button Assembly Replacement - iFixit Repair Guide

You might compare your work to that for the iPhone X; on that one there are four different lengths of screws used so you might try comparing yours with those on the X and see if there's any correspondence there.

Have you done anything with the exterior buttons? Those could also affect whether the internal buttons are working or not. If you didn't remove or disturb them, then you're probably okay, but there is a guide to replacing those as well on the iPhone X. Again, this procedure will be identical to your phone, at least with respect to actually removing and replacing the buttons themselves, although the procedure of how you get to them will be different.

iPhone X Power and Audio Exterior Buttons Replacement - iFixit Repair Guide

I hope this helps; feel free if you have any questions I may be able to answer since I wrote those guides.