Security warning first: this copy of Volume & Sound Booster contains malware
Do not install this XPI. If it's installed in any of your browsers, remove it.

The real volume feature is the ~150 lines in cs.js. The other files (csvolume*.js, amplify-relay-bg.js) are pieces of unrelated libraries (Dark Reader, AdGuard scriptlets, lodash, Vue) with renamed functions. That filler hides a remote-control loader:

Delayed start: amplify-relay-bg.js:770-790 saves an install timestamp and does nothing for about 3 days (laneGapMs = 258200000). This is a common way to get past store review.
Hidden server address: csvolumepatch.js:808-812 calls cloudflare.com/cdn-cgi/trace to get a value h. That value is part of the key that decrypts the server URL, so the URL can't be read from the code alone.
Network rules from the server: amplify-relay-bg.js:825-840 downloads a payload and applies it with declarativeNetRequest.updateDynamicRules. That lets the server redirect requests or rewrite headers on any site.
Remote code in every page: csvolumepatch.js:850-890 runs code from that payload inside each page, using new Function(body)() or a function exported into the page via wrappedJSObject. It skips pages that contain a certain "guard" string.

Worked pretty well on youtube.

Doesn't even install the right extension, this and the other volume control extension both add the exact same extension that is a different one. I don't know what this is but it feels weird because it doesn't give me what it should give me

The volume boost is noticeable without being complicated to use, and it has helped me hear videos and streams much more clearly. Very useful and easy to use.