Help joining two objects together seamlessly

Hello I was wondering if someone could give me some advice on how to do a thing in blender

I created a human head model in FaceBuilder add on and I have a separate premade human body model. I positioned, scaled and joined them together, but there are some small gaps between the two models

What’s the best way to fix this? Any tools or tips to get me moving in the right direction would be amazing

I’ve been researching the docs, YouTube, chatgpt but getting confused at all the different ways to do things and the different versions of the program.

I’m using blender 3.5

Thanks!!!!