How to save data from html (form register save user)?
It will require a backend. To make this, you have to learn any backend language like python, Java or Php. If you want to make a backend immediately within 2-3 days, just learn Firebase. Firebase provides free databases to users. Even my website https://arnavgumber.github.io/codeinfinitewitharnav/ uses firebase database.
Thanks, I will try it.
Collect and send users' data to my website
Do you mean "save data to user's computer" or "collect and send users' data to the website owner". Tell me, I will answer it