Why should I download YT Revanced from here?
For speed/convenience because you don't need to patch it yourself. That's literally the only reason to get a pre-patched apk from here.
revanced.net
Not the official site, do not use. The only official site is revanced.app. You can also get everything you need from their official GitHub github.com/ReVanced. Their GitHub also lists their official site so that's how we know that revanced.app is the real one.
is it the same thing as on Mobilism?
No. Mobilism uploaders provide a pre-made, pre-patched app for your convenience. With Revanced Manager you get more control but it's more technical and takes more time to set up and stuff. You can choose your own patches/features and can patch not just YouTube with Revanced but a plethora of other apps too. That's the difference: getting a Revanced app someone already patched vs patching your own. The Revanced team do
NOT provide ready-patched apks, only the Revanced Manager app github.com/ReVanced/revanced-manager which does the patching and their version of MicroG GmsCore github.com/ReVanced/GmsCore for Google account functionality on non-root devices
Revanced Extended
Extended is just additional patches, some of which are experimental and are later added to official Revanced once they're stable, that are provided by another GitHub user (they also work on the official Revanced so they're trustworthy). You can use Extended patches in Revanced Manager by changing a few settings.
