Mac creates echo for video call partipants in speaker mode

That is to be expected when you use the built-in mic and speakers. The mic picks up the sound coming from the speakers then re-sends the sound which is the "echo" being heard. The solution, as you have found, is to use and external headset which the internal mic can not hear and resend.

You can try adjusting the microphone input sensitivity to be less sensitive so that it may be better able to ignore the audio from the speakers and only activate when you speak. Some of these apps have this adjustment available within the app itself....if not, then the macOS Sound Input System Settings has a slider to adjust the microphone sensitivity. It also helps if you lower the speaker volume as well.

Plus keep the audio muted until you speak.

However, it is usually best to use ear buds or headphones as you have discovered.