I was looking for the source code of DAVdroid, but I had to resort to google (which got me to gitHUB, which in turn got me to gitLAB).
Places I’d look for first:somewhere near “android app on google play” on the start page somewhere near “Open Source GPLv3” in the feature list on the start page maybe in the FAQ (“Where can I get the code?” or “How do I contribute?” would be a nice addition) I didn’t search for it on the download page, but this would also be a good place, I think
I’m not saying all of these place should have a link to the source, but one of those places probably should.
The links are in some weird footer, that looks more like an ad. I completely ignored that part, especially since it’s sandwiched between a bunch of logos and some “Legal Disclosure” stuff.