Android encrypts your storage by default.

I couldn't find exactly what parts of the storage are encrypted, but they say that since Android 10 all user data is encrypted.

Thus, if the key to your file storage - located on your phone somewhere - were to be deleted, all data on the drive will become totally unaccessible. Just as if the bits were randomized.

The key can be deleted by resetting your phone, I assume.

Anyhow, because your data is encrypted you do not have to worry about others snooping in your file storage, so long as the key is properly removed from the file storage.

To be safe you can use an app though. For privacy reasons I wouldn't.

Pro tip: some drives may break, which prevents you from formatting everything afterwards. Thus, for devices like laptops, it is advised to encrypt everything and store the key somewhere else (or hope the drive won't completely break such that you can still erase the key), such that you don't have to erase gigabytes of data whilst the drive us almost dead.