I’m not the best modeler on earth but I would probably try to merge vertices here, so make sure that your objects are one, get the vertices close together and then merge them (tbh I’d also need to Google where the specific options are but I do remember doing similar things so it’s def possible).

If you want advice on getting better:

I’d start by paying close attention to the number of vertices/faces you are using. You have way more than you need for a basic topology and the number of edges is not even remotely similar between the two objects which is troublesome. They don’t have to be exactly the same but you want a ratio that makes connecting them nicer

When you have less vertices, you will find it easier to just manually connect the two objects by creating faces (pick the edges you want to connect and press F)

This really gives you a greater control over your work and will be really beneficial in the long run. Hope this helps!

I would use a boolean modifier set to union, apply it and use the sculpt mode to smooth the intersection. This will ruin the topology so if you want to animate it you will need to retopologize.

Alternatively you could merge the vertices as was suggested, or just keep them as two separate meshes and use clothing or necklace to hide the neck seam.

Shrinkwrap modifier on the top edgeloop