My project has a very low resolution, however, I want to make some elements with higher resolution so that they don't look pixelated, like the GUI. How can I make this?
You can do this by creating a Viewport node with a smaller resolution (but stretched to cover the whole screen). This means you effectively have two viewports in your scene, since the root node is already a Viewport.
See the 3D viewport scaling demo for a working example (the same principle can apply to 2D as well).
What I want is the opposite, a viewport with higher resolution than the rest of the game.
instead of changing zoom globally in the project settings make a couple parent viewports (and related cruft) and set the zoom individually on both (ViewportContainer has the setting) so each is at an appropriate level i think https://imgur.com/GCjKvt6