The canonical answer to “how do I split windows” is: use your window manager from the operating system! What you’re looking for in practice could be a simple window arrangement away.
macOS got better at this but you still probably need a 3rd party app to make it easy to arrange two windows next to each other in a tasteful way, or have them ‘stick’ to each other. Sad!
But I admit I’ve personally thought too much about split panes myself even though I have all the power tools and customizations in the world. Having 1 window “box” around everything is, if nothing else, more comfy.
Please don’t tell the rest of the team, I may be lobbying for this over the next 10 years in a subtle way.