When using bitcoin-qt wallet, all commands are executed externally with bitcoind?
I am asking because I saw an altcoin wallet-QT (DMD) which comes as a single file without any daemon file, does it mean the daemon is already built in the qt wallet?
Bitcoin-qt does not run bitcoind as a daemon. If you check the running processes after you launch bitcoin-qt, you will not see bitcoind being launched.
They both utilize the same "Bitcoin Core" source code obviously, but bitcoind and bitcoin-qt are separate programs and one does not need the other to function. You can think of bitcoind as a GUI-less implementation of "Bitcoin Core" and bitcoin-qt as the GUI implementation of "Bitcoin Core".