inside of clothes mesh invisible.

- select the reference, disable Limit selection to visible, select everything that is covered by the garment, delete it.

- join the garment with the body. Select everything, Select - Select boundary loops, Mark as seams;
- Alt click to select loops in places where the body is meant to connect with the garment, press the space bar and type Bridge, choose bridge loops. Do not worry about the uvs for now. Do this in all other places the body is meant to connect with the garment.

- once done, select one face on the created geometry, Ctrl L to select the rest of it, choose Seam, unwrap.