My heated bed is warped quite a lot and I was looking for the best solution I've read about borosilicate glass but do I just mount it to the plastic in place of the other glass or do I mount it on top and I also have a similar question for alumininum heated plate do I mount it on the plastic. I just feel like the plastic is what's warping so will the borosilicate stay flat enough not to warp. If you have any other solutions I'm open to them thanks in advance for any answers/help.

The existing glass is only about 1.5mm thick so it deforms. Boroglass is 3mm and is much more rigid. Just mount it on top of the existing glass under the existing clips.. Metal beds are fine for a  primary surface but you will still need glass, kapton tape or painters tape over it.