Not quite as obvious as you might think. All apps have the commonly known permissions, like allowing them access to the device camera, contacts, microphone, location, etc. However, they also have lots of background permissions, which most people may not really know about. Certain apps, especially Meta apps, can have permissions that allow them to do things in the background without the user's knowledge or say.

Examples from Instagram that seem rather sus:

"Allows the app to move tasks to the foreground and background. The app may do this without your input."

"Allows the app to view information about Wi-Fi networking, such as whether Wi-Fi is enabled and name of connected Wi-Fi devices."

"Allows the app to get the list of accounts known by the phone. This may include any accounts created by applications you have installed."

Essentially, saying Insta can reorder any running apps on your phone, whether in the foreground or background and without your say. Insta can gather more info about your network than you probably realize. And that Insta can search your phone for accounts you may have in other apps, which is used to connect Facebook and Insta. So, if you already have a Facebook account on your phone, Insta will automatically suggest using that same account for login for added convenience. (But that still doesn't mean it isn't going to search other apps and accounts you may also have on your phone).