Different portals go to one the same portal in Nether. If I build first portal, it create portal in Nether. If I go into another portal to Nether, I will appear at the same place where I go from first portal. When I go back to this portal, I will be at first place in my world (where was built the first portal) . It seems to be that all different portal in world go to the same place in Nether. I tried it in some my worlds. If I destroy this first portal in my world, then I go to Nether from another portal and will go back to world. Then I will appear in the same place where was the first portal and game creates this portal nearby to destroyed portal.