I have had the privilege of interacting with some true experts. Professionals. And insanely smart people (Nobel prize winning level). The level of knowledge seems to be inversely proportional to what they purport to understand.
Meaning the smarter or more knowledgeable they are, the more they highlight just how little they know. Ultimately all they talk about is just how little understanding there is on complex/difficult subjects. And they include their own limited understanding in that.
Which word is used (expert vs professional vs guru vs zen master... doesnt really matter imo). One of my fav quotes:
The fish trap exists because of the fish. Once you've gotten the fish you can forget the trap. The rabbit snare exists because of the rabbit. Once you've gotten the rabbit, you can forget the snare. Words exist because of meaning. Once you've gotten the meaning, you can forget the words. Where can I find a man who has forgotten words so I can talk with him? ― Zhuangzi