So I mostly stuck with really simple text editors (first likes of notepad++, geany and gedit, later switched to barely customized vim). You learn the language, you learn its standard lib, you learn the libraries you use often, you learn to navigate their docs. You learn the project you are working on, and remember how things are named. I do use a simple autocomplete (ctrl+n in vim), but it's more of a typo preventer (or a typo propagator, depending on how you look at it). It autocompletes every word in open files. Which might be more handy than it sounds, because it will autocomplete stuff not typically being autocompleted in IDEs, like json keys, or file names in the open directory listings, or even outputs in the open terminal session.
As for navigating unfamiliar code bases and "go-to-definition": it's grep. Just search for a substring in the whole project. You will find the definition. You will also find some other interesting stuff, which "normal" IDE tools wouldn't look into. Heck, you'll find interesting comments, interesting name clashes, interesting usecases for a thing you were looking for. And it's a language agnostic skill. You don't need another bespoke IDE, you don't need to configure some weird LSP to navigate unfamiliar code base even in not so familiar language.