freelancer Yes I've read [it], still leaves me having questions
Which questions?
Something to keep in mind is that the way Android works is actually complicated, so any short simple answer probably leaves things out. In this case the only short and simple and accurate answer is probably "Yes, apps can find out which other apps are installed". Moving beyond the "Yes" to things like "Yes, but you can ______" will inherently be more complicated. Coming to an accurate understanding may well require reading a bunch of text and asking some specific questions about that text. It sure would be convenient if the answer were "No, apps can never find out anything about other apps", but Android doesn't work that way.
Most apps can't learn the phone number of an installed SIM card, but some can. And all apps can learn the country code of an installed SIM card. Apps are allowed to know the local time, hence the time zone, which for some countries narrows location down a fair amount.
Unfortunately, the problem isn't that the documentation is complicated or that the answers on forums are complicated; the problem is that Android is complicated.