How to mirror head geometry? For example, I have a human head model, which is cut in half, because i want to mirror it. But the problem here is that some faces are over the Y axis and some do not touch the axis. When I then try to mirror this shows up, where let's say some points don't touch and other ones, where faces are over the axis just collide. Is there a possibility to align the "edge" faces to a certain axis, so the mirroring becomes perfect?
In the mirror modifier check merge and clipping : this will automatically aline and merge the vertices

change the merge limit if there is some gaps

you can also select these vertices and hit S+x then 0 so they aline on the Y axis