Is my phone encrypted?

Yes because the FP3 is encrypted by default even if you don’t set any password/PIN.

But according to what I know it is stored in a secure environment and even if you are rooted you can’t just access any “decryption_key.txt” file. Same thing for iPhones for instance.

Of course not a single device is perfect and I’m not an expert so I can’t give more details.

Never heard of such a feature.