So How exaclty do you get the best performance out of a game in unity

So ive been working on a game for about 2 months and I dont know if its the code or the UI or the objects thats making me lag but i really need to know the most simple and efficient way to clear up laggy ness and make it run smooth without lowering the graphics because it also still lags when i do that to. Is there anyway to add time . delta time to everything.

I also want to know this, but I have a few solutions already. I learned that you have to take a look at your textures and change the quality to mobile as example for less lagg, you can also use '‘Occlusion culling’ to reduce the lagg. It only shows objects seen by the camera. Next to that you can also just put the quality down, like the aa or the shadows as example.

You use the profiler.