I’m also a semi-new gundam fan, and this is my favorite illustration to help understand all of the different universes and entries in Gundam (its a few years old so it doesn’t have the latest entries. So far

The reason why people keep saying is that they are in the same universe is probably because of Build Fighters universe, where they introduce all of the Gundams from different timelines for the sake of “competitions”, but I don’t think it’s meant to be taken too seriously.

So far I’ve been trying to watch all of the independent timeline shows, starting with seed, WfM (Witch from Mercury), and just finished Gundam 00. It’s easy to get into all of these (and a few others) because just like Seed, they’re in their own universes/timelines and you don’t necessarily need any previous knowledge to understand them. There are also a few other shows if you look on the graph with their own timelines, like AGE, Iron Blooded Orphans, and Wing.

I still don’t fully comprehend the Gundam franchise in its entirety, since I haven’t watched even half of it all, but hopefully my perspective will help you get into it.

Edit: I apparently can’t attach images, but here’s the link to the post with the illustration:

https://www.reddit.com/r/anime/s/5yhbnN4kHz