Do I need to create a child theme? And if so, how do I do it?

I am working on several WordPress websites. I am told that I need to create child themes for them in order not to lose the edits I make when the themes are updated.

However, if I use Jetpack to edit the CSS, then I understand that I don't need to create a child theme.

What if I edit the php or the html? Do I need to have a child theme set up so as not to lose the changes? If so, I need to be walked through it. Because I can create the folder, but I don't know what to put in it.

I've read lots of instructions on this but always get lost after the child theme folder is set up.
Thanks for your help.