Your browser has a chronological list of the previous pages you visited. It’s very easy to quickly go back to a page you had open recently and fairly easy to search for it in the history if it was some time before. It’s actually probably easier than finding a tab amongst a lot of tabs.

For persisting state, it’s simple. You don’t. History is going to send you back where you were if the url scheme is not brain dead. I will personally stop using sites which don’t do that properly because they are annoying to use.

For forms I just fill them when I need to or keep them open for a bit if I forget I would need some information which I need to check. If I realise it’s going to take some time, I just close them and fill them properly when I have all I need.