

A longer answer to those comments already here is that it may depend on the particular app. If you are trying to prevent the browser or a system app (including perhaps the app store or assistant, among others) from being opened, it may not be possible, depending on the exact OS you are using.
Most other apps can be prevented from opening links, as described in other comments.
Yes. Most of those settings are unlocked and you can choose your own defaults. One of the major appeals of using GrapheneOS imo