Load content according to the device

I’m making a responsive webapp, that is, it will work on mobile and desktop.
But how can I upload separate content for each version?
Example: I have a fixed menu at the bottom that works on mobile, and I want it not to load with the page on the desktop. Note that I don’t want to hide the menu, but not to load it on screens of 1000px or more.

Example two: I have a full screen menu on the desktop, I don’t want to load it in the mobile version.

How to do not to load an element at a specific screen resolution, thus making loading faster, loading only what is necessary for that device?