For example, Logseq has a fancy text field that can bring up a submenu if you type two left brackets. Something like this is pretty specific to Logseq (or at least certain notes apps) and this would be much harder to replicate in a native app.

Not something I would consider terribly hard to implement, but it would depend on the toolkit. A function for getting the text in a textbox and a callback to alert you to the fact that the user is typing is something I would expect to find in any modern GUI toolkit.