lens questions

I'm new to flashlight modding but I caught the bug hard. I have been ordering stuff left and right and im trying to learn a little about lenses. Most specifically these optic lenses im seeing everywhere. For example I see this one on illumination supply https://illuminationsupply.com/carclo-10048-265mm-plain-tight-tir-p-173.html . What exactly does it do? Im thinking it would replace the reflector on a p60 host and make the beam tighter compressing all the light from an xml into a 15.2 degree beam. I kind of see how that one works since its about the same size as the p60 reflector so it will fit. How does something like this work http://www.fasttech.com/products/1108201. Would you cut down your origional reflector, sit or glue it on top of this and tighten the lens against the top part of your origional reflector? This one too, how would it be used http://www.fasttech.com/products/1108201 ? One thing im trying to do is make the beam tighter on a C8 light that I have, it currently has an xmlt6, i also have another pill with an R4.