Is makeup fixer used before or after makeup?

I'm curious to know, when it comes to applying makeup fixer, is it typically used before or after the makeup process? I've heard differing opinions on this and I'm eager to get a clear understanding of the best practice. Could you please clarify this for me? Is it more effective to use the fixer as a primer, before applying any makeup, or should it be applied as a finishing touch, after the makeup is already in place? I'm looking forward to your insights on this matter.