I would say, first of all, that you shouldn’t stress too much about it. I think it’s more important to keep a healthy peace of mind rather than fighting for every last bit of privacy on your phone.
That said, you can find out which apps contain trackers with Exodus. Read through the website to learn more about what trackers are, how they work and how to avoid them.
A firewall app will allow you filter connections and record them to see which app is making them, and even block them. There are a couple of discussions on the topic using different apps: NetGuard and TrackerControl.
This is another install and forget ad-blocker you might find useful instead.
Note: I don’t think you can combine these apps together. Learn about how each works and try them for yourself.
Regardless of whether an app has trackers in it or which app store it comes from: restrict the permissions to those that are actually needed. As far as I’m aware you can’t choose which permissions to grant during installation, so you should do this immediately after. Of course this may sometimes break functionality and you should make a decision on a per-app basis, but the fewer permissions the better for your privacy and security.
You can also isolate apps with Shelter in a separate profile than the rest of the system. It will give you a little extra privacy for those apps that you absolutely need to grant sensitive permissions. There is a detailed guide to use WhatsApp under Shelter in the community that you can use as an example.