Do you lose custom pages in updates?

I know I have to make some little code changes for every update, but it takes me ages to make custom pages (without sidebar) in html and to ftp them.

Do I have to do all that again for each update?

Also, my custom templates seem to have disappeared, so I can’t quickly go and reshow the pages. I heard something about ‘twin’ or ‘child’ sites to overcome this(?)

What is the best way please?

thanks a lot