I am building an app that requires a user to fill up a form at some point. The app would of course have a back button and hamburger. Should the universal back and hamburger disappear when the user clicks into the form(the form will have its own back button)? Or should they stay where they are and users can bring up then nav even in the middle of the form?
As a general design principle for the web, you should provide a way for the user to navigate your system, which includes always providing a way out as a must. Ok, users could use the native button, but it's always better to be explicit since you should design for everyone and not expect users to resolve a simple situation as going back or navigate to other section/page.It's to always keep in mind the title of a famous web usability book "Don't make me think".
A case that shows that they are necessary is that users may have to do something relevant before filling that form or to perform an action that is necessary to complete the form correctly, e.g.: a form that contains a category, so when you open the form you remember "Oh I didn't the necessary category yet, I will do it now => leave form.)
Also it's a matter of consistency. Menus, including hamburger exists to provide some kind of "index" where users know securely that they can reach if they get lost or if they just want to go elsewhere. As an analogy imagine you are a tourist in a city you don't know . Then you enter a big museum, where they give you a museum map at the entry. Now imagine that when you reach a particular section of that museum both the museum map(navigation) and the exit signs(~back button), just disappear without reason. Visitors won't like that and here user won't either.