Coinbase address still has a balance
Coinbase (the company) is not bound to any value presented in any block explorer or even in the actual blockchain.  They decide what will be presented as your balance on their service's website, and hopefully it is always the same value that you expect to see.

Coinbase might allow funding of your balance through payment to bitcoin addresses over the bitcoin network, but in reality they will maintain a separate, much more efficient database than a blockchain to track that balance from that point on.

When you operate on those funds, you're operating on the number kept in Coinbase's database, not the one in the blockchain.  It costs money (as fees paid in bitcoin) to move funds between participants on the network, so any transactions that can be kept off chain are done within their own database (like sending between two Coinbase users).

In case you are withdrawing bitcoins from Coinbase, they will have to use an on-chain transaction (eventually), but the funding to it can come from any other payment made to them.  It doesn't have to be the same payment that you made.