You need to show someone how to do something, or report a bug, and you do not want to install a recording program for it. Your browser can already do the recording.
Close anything you would not want in the video. Notifications, messages and other tabs can appear while you record, and sharing only one window or tab keeps them out of it.
To include the sound that the shared tab plays, use Chrome or Edge and tick the option to share tab audio when you are asked. Your own voice is added separately from the microphone setting.
The recording is kept in memory until you save it, so a very long one needs plenty of free memory. For anything over about half an hour, record it in parts.
No, and there is no time limit set by us either.
No. Phones do not let a web page record the screen, so use a computer.
An MP4 where your browser can make one, otherwise a WebM. Both play in most video players.