Comment by btown
IMO this is one of the most important skills for a developer to have. In an age of Github Copilot and similar systems, it's both far more viable (because, per OP's #1, you won't get distracted by the "side quests" of implementing utilities you need, when you can just tab-complete to get them) and far more vital (because with that productivity increase comes an increase in the complexity of problems you'll be asked to tackle at any given level of seniority).
My advice on this would be: never be afraid, and even force yourself to, follow the chain of how a certain function is implemented in third-party libraries you are using. Set up your IDE to be able to click into functions, make that muscle memory, and perhaps even have a dedicated project you can bring up for your site-packages or node_modules. Don't just rely on documentation, see how things actually work a few levels deep. You'll gain a deeper understanding of the code you're using, as well as incredible practice for when you need to do this on an existing first-party codebase.
Oh, and if you can, get one or more large 4k monitors, and split your IDE into 4 quadrants! It's certainly possible to hold a codebase in your head on a small laptop screen, but being able to see the code you're working on alongside its dependencies and dependents makes this far easier!
This is one of my standard interview questions, how do you familiarize yourself with a multi-million line code base with partially correct partial documentation (in addition to generated documentation). Looking for interesting tooling, and stepping thru the framework code to see how it all comes together (and enough concrete details to verify the claimed work history).
I'm not sure about the advantage to seeing all the code at once for understanding it. Surely for refactoring or massive editing, but to encode it into your brain, I feel the crucial thing is more abstract than actually seeing the code.
I personally find the large screen is something that's more useful when I am moving between different low-concentration tasks (confluence, slack, teams, emacs, whatever) but for deep problem solving/thinking, just 1 window I can see everything completely with a full-screen emacs is more conducive. I assume other editors than emacs would have this same facility. Extremely rapid answering of different questions about the code via LSP type facilities is more key than seeing a bunch of text at once, so I can follow the flow of my thoughts. Sometimes, when I'm trying to make sure that 2 or 3 functions have the same flow/logic (yeah, haven't found the abstraction to replace them with) I'll want a big side-by-side comparison of 2 or 3 places at once.
This ability to follow into third party code you use was one of the initial attractions of Gentoo - you can build all from source, keep source and symbols around, and edit/trace the code for anything on the system. Reading a lot of code I think makes my own code better.