Not using the weight painting brush with symmetry on, did it instead in vertex mode, how to mirror?

Rigging Fingers:
I go to vertex mode and manually select the vertices in the viewport and then go to the vertex group and select the group they are not suppose to be in and clicked “remove”.

I did this a couple of times until the fingers are rigged perfectly and moving the first finger doesn’t affect the second finger.

Now how do I mirror that weights ?

You can mirror the selection but only when your model is symmetric. I search for this operator with F3 i think (its called mirror selection).
When you dont have the selection anymore just select all the verts of the good side and mirror it, then invert the selection with CRTL+i and remove that from the vertex group.