You record either your screen or the camera. The button is available on other pages too, so you do not have to come here first.
Recording
Starting has to be a click: the browser only offers the choice of which screen or window to record straight after something you did. That is why there is no countdown and no remote start.
A longer take is split into blocks. That is deliberate: the whole recording never has to sit in memory.
After you stop
If you recorded one block it stays in this tab and the editor opens on it. The recording then exists only there — close the tab and it is gone. That is why the editor has a button to download the untouched file.
If there were several blocks they download one by one: the editor takes a single file and the browser cannot join them.