A few years ago I sent my BTC from Coinbase to Electrum on windows PC. That apparently went fine because Electrum showed the correct balance. Then I received 12 keywords and a hash (starting with 17) which I always thought was my bitcoin key. I saved these on paper and deleted Electrum, thinking OK I have all I need on paper.
Now want to transfer my BTC back to Coinbase. I entered my 12-seeds in Electrum and the balance was right. I hit {Receive] in Coinbase and I get a window giving me the Coinbase(?) address to send my BTC to from Electrum. But this address is the same as what I thought was my private key (starting with 17). This is weird especially because 1) How would Coinbase still know this number and 2) Electrum says [Pay it] to (what I thought was) my private key. What am I missing here. I don’t have many bitcoin but the price is too high for me to make a mistake and get an education.