Found 0.2 Bitcoin that i bought a long time ago, but forgot how it works

About 10 years ago, I bought 0.2 Bitcoin out of curiosity, and recently I happened upon the document where I wrote down the information about it. There are a few user names and passwords that I don’t quite know what they are good for, so I installed Bluewallet (iOS) which asked me for a seed. So I entered this “seed“ phrase, which seems to be correct, and now I can see the balance (0.218797) in my BlueWallet wallet.
1) I guess this means that I could sell these 0.21…. Bitcoin if I wanted to (I don’t want to, it’s hypothetical)? Would I need a password? Im pretty sure i have the correct password, but how can I test this without actually selling anything?

2) I don’t understand where my 0.2… Bitcoin is stored. I think it’s not stored on my device, otherwise I wouldn’t need the seed phrase to access it, right? So how can I know where it is stored?

3) (Related to Q 2): If I wanted to buy a little more, how could I add that to my existing balance rather than creating a new wallet?

Thanks in advance