You seem to be confusing two different security features that are intended to protect against two different types of attacks.
The three attacks you mentioned are to bypass the lock screen of the phone, and have nothing at all to do with encryption. They also only work in a specific set of circumstances.
Among other limitations:
- The phone must be already booted up. In order to do anything useful, the phone's OS needs access to the disk and therefore it must have the decryption key loaded in memory. Several of these types of attacks will not usually work against a phone that has only just been booted up but not yet unlocked for the first time after booting.
- The phone may have a bug. In your third example, bypassing the lock screen could only happen because someone made a mistake. Programmers are human too and they make mistakes. It sometimes happens that mistakes allow a security bypass.
- You have chosen to deliberately reduce security. In order to use your first example, the device owner must have enabled debug mode. This is not an easy thing to do - there are deliberately multiple complex steps that must be taken to enable it. You also need to allow each specific computer to use ADB, so this attack can only be done from a computer that the phone was previously connected to. Or, in your second example, you as the device owner have deliberately enabled a bypass of your security, trading it for convenience in case you get locked out of your device.
So what does device encryption do for you?
Device encryption protects your data from being accessed from outside the OS.
- An attacker cannot reboot your device into "recovery mode" or de-solder the flash chip from the motherboard to read your data.
- Every time your phone is rebooted (not just turning off the screen; only a full reboot), you must unlock it once before any data can be accessed. This is because Android encrypts the disk encryption key using your password/PIN/pattern, and the OS cannot access your data until that first unlock. (One way you can see this easily is if you reboot your device, then leave it locked and have a friend call you. Even though the friend is in your contacts, the caller ID will only show their number, not their name, because their name is stored on the not-yet-unlocked encrypted storage.)
If you want to add another layer of protection, some Android devices will allow you to add a password to the disk encryption. This adds an additional password prompt while the phone is booting up, and the phone will not boot until you have entered the password. However, unless you are afraid of nation-state level attacks against your phone and you plan to always keep your phone powered off and only turn it on when you need it, it is highly unlikely to be worth the trouble to set this password. (Again, remember the difference between disk encryption and the lock screen.)
