>Why exactly should I trust any of these to have full access to my browser?
Content blockers on iOS don't have "full access". Most adblocking apps provide both a content blocker and an extension, the latter of which is used to work around stuff that content blockers can't block, or bugs that result as of blocking scripts from loading, but they're not needed. You can get 95% of the functionality by just using content blockers.
I guess my head is still spinning.
I took a second look at ad blockers on the app store, and many report that they collect various bits of data. Are you saying that there's a special content blocker component to all of these that can't collect data because they're isolated by iOS? I'm not sure how anyone who isn't a iOS developer is supposed to navigate this. To uBlock's credit, their App Store page reports that they collect no data, but is this enforced by iOS? Or just a checkbox that the developer clicked?
>I took a second look at ad blockers on the app store, and many report that they collect various bits of data.
Because the "app privacy" disclosures that apple only contains broad categories about what data the app can possibly collect. If the app collects analytics in the UI itself (ie. the part where you select filters or whatever), it has to say the app collects analytics. It's not possible to say "we only collect analytics on your usage of the app, not what your browsing history is".
>Are you saying that there's a special content blocker component to all of these that can't collect data because they're isolated by iOS?
Yes.