This is a big repository of user provided packages for many-many programs and their dependencies.

Basically it works like that. You download the build script, extract it to a directory, you download the source (or in some cases a different package format, like rpm) into that directory, you run the SlackBuild script as root, which makes a Slackware package, which you then install with installpkg. Simple, done. Of course that's only if you don't have to take care of dependencies, than you will need to do this many times. The good thing you can make backup of your packages and use it later.

Just look at SlackBuilds and follow the instructions. Many times it has package specific instructions.

Just like in any other distro, multilib means 32-bit and 64-bit support. While the world is moving towards 64-bit, there are still many software that is 32-bit. Hell the gaming world is still clinging to it.

I followed these instructions: https://wiki.alienbase.nl/doku.php?id=slackware:multilib

You need multilib enabled for Wine and Steam too. Just go through the steps from the above link.

Then create Wine package from SlackBuilds. b