> What if you have a modal opened with some state?

Stimulus controllers can store state.

> Or a form filled with data?

Again, you can either use a Stimulus controller, or you can just render the data into the form response, depending on the situation.

> Or some multi-selection in a list of items that triggers a menu of actions on those items?

So, submenus? Again, you can either do it in a Stimulus controller (you can even trivially do things like provide a new submenu on the fly via Turbo), or you can pre-render the entire menu tree server-side and update just the portion that changes.

None of these are complex examples.