How to join two mesh in one go

Hello everyone I am trying to join two meshes into one without any manual way . Is there anyway to do it easily and in one go??
Thankyou for your help :blush:

Select them both and press Ctrl J? :wink:
If you mean you went manifold geometry, do that and then go into edit mode, press M and choose By Distance. Turn up as needed. If that doesn’t work, use a Remesh modifier

It’s very dense. Maybe you could dissolve some edges on the piece with more segments. ( select edge >> X key >> choose “dissolve edges”

“Without any manual way”? I don’t know of any, though there could conceivably be an add-on. Or do you mean, without you manually connecting each vertex on one mesh to one on the other mesh? That can definitely be done.

Ideally your meshes would have the same number of vertices where you want to connect them, then you can do it “in one go” – but yours don’t. Still, this is done quickly by selecting the two edge loops and doing Edge → Bridge Edge Loops. You can also press F to create faces between them, which gives a different result (IME not always an acceptable one). Your new topology might be kinda ugly, and you’ll need to clean that up. So it might be better to reduce the geometry of the denser mesh first, if that can be done relatively easily.

To use the Merge By Distance method, your meshes need to be quite close together or you’ll be merging a lot of vertices on the meshes themselves rather than just between the two edges.