We are going to need more people mixing songs so we can work through all the submitted vocals in time for the deadline, so I've prepared a resource of torrents for plugins you can use to help you get started with mixing your vocals and stuff later on, I pulled all of them from RuTracker: https://files.catbox.moe/7lhn47.zip
It includes a mixing template for REAPER and a short guide for some of the plugins, and a short guide on how to use some features of REAPER.
Use Qbittorrent to get the torrents to download. Torrents are basically download links, so just having the "torrent file" won't give you the plugins.

For audio software, I recommend REAPER. It's not as easy as Audacity, but once you learn it you'll be able to do whatever you need: https://www.reaper.fm/download.php
Recommend looking up some guides for getting started with mixing, but worst case you can ask in the thread.

In case you have a powerful enough PC, this is what you can use to remove vocals from songs you've downloaded: https://github.com/Anjok07/ultimatevocalremovergui <--- USE THIS FOR INSTRUMENTALS

In case you need high quality files for songs you like, the best bet is to find them using Soulseek: https://www.slsknet.org/news/node/1

And finally, in case your music only exists in the form of a Youtube video, here is YT-DLP. It's CLI, but it works well enough if you remember to use "yt-dlp -x ": https://github.com/yt-dlp/yt-dlp
YT-DLP has a few GUI implementations, such as TarTube: https://tartube.sourceforge.io/