how to download itunes

There is none. There is no longer a version of iTunes that works on Catalina.

All its features have been split up betweeen the TV App, Music app, Podcasts app and the Finder.

Is there some feature or functionality you are looking for specifically?

Start here: What happened to iTunes? - Apple Support

As far as I know, there's no iTunes on Catalina, it's "Apple Music", and 2 other apps.

You may find some hacks somewhere, maybe, but the two softwares will be messing with each other.