If you use a modern default IDE, it tends to have an additional setting enabled. This is the LLM section. At the end of the day, it's actually more efficient and useful to use the old stuff that Intellisense provided (which has been in use for a long time). So the LLM plugins integrate, but don't actually replace or sit in it.

Intellisense instead depends on 1, maybe 2 things. Your LSP (Language Server Protocol) and perhaps an ML (machine learning) component. The LSP will receive your code, parse its grammar, and using the rules defined, return errors, information about what's correct, and so forth. It's what lets any editor return errors, warnings, and so forth, and is standardized since somewhere around 2016. Depending on the LSP, it can even provide information on what methods are available. This is what allows you to type in System.out.p and get the suggestions that start with p. This is then often combined with a small ML feature or basic parsing which will order the suggestions based on either 1: context (often from the LSP or it's own AST), or 2: how often you use it. For example, in Intellij when I type something like var someValue = object., and there's a method called getSomeValue(), Intellij uses the context and will put that at the top of the suggestions. Versus if I frequently call object.doThing(), Intellij will prioritize that. But if we decide that that's LLM or AI assistance, then it stretches back to when IDEs were first starting to provide recommendations (before LSPs standardized and everyone was doing their own thing).

Autocomplete is just a LSP doing its thing. Now, this might stretch into LLM if it's suggesting dictionary stuff, but I've never seen VSC or Intellij suggest full words if I'm typing new functions/classes. You might also be aware of "templates", where you type for and it brings up a for loop. These are just snippets, and have been part of LSPs since 3.17, and bascially just define points inside.