Unity Scaling Entire Game Build to different aspect ratios (Not canvas scaler?)

Unity Engine
2

As you intuit, 3D is gonna be different than 2D is gonna be different than Canvas UI.

To keep important stuff in sight, I would just reach for Cinemachine.

Here are the official docs on how to make your UI handle various resolutions and aspect ratios cleanly:

Here are some notes on UI Anchoring, Scaling, CanvasScaler, etc:

Usually you need to choose a suitable ScaleMode and MatchMode in the Canvas Scaler and stick with it 100%. Generally if you change those settings you will often need to redo your UI entirely.

Here’s some more fit-inside-UI-content notes:

https://docs.unity3d.com/Packages/com.unity.ugui@1.0/manual/HOWTO-UIFitContentSize.html

I also use this CanvasScalerOrientationDriver utility to make sharing UI for Landscape / Portrait easier. Read what it does carefully.