How do you implement the automatic saving function in the middle of the HTML input form?

The stack overflow form data is taken over even if you change browsers or terminals, so the server is probably saving the data.

You can also use cookies to save things like "Omit Login ID from next time" which are common.
If you want to save it dynamically in case you close your browser unexpectedly, you may need to combine it with Javascript (Jquery) and so on.

I think the implementation method will change depending on where you want to store the data.