If you are teaching via Google meet, then use Google Jamboard it can used as a white board, sticky notes are available etc.
If you are working in a unix operating system, then XOurnal ++ is a wonderful to take online classes. For windows similar operating systems are available like xournal, Jarnal etc.
If your institute is ready to pay, then prefer ZOOM as it contains many tools to make class interactive. It also contains a feature of breakout rooms to distribute students in to different class rooms and they discuss among themselves. Here one can annotate PDF files.
Simple Screen recorder is simple software to record your screen.