MacBooks have terrible thermals. They will thermal throttle before most laptops, much less even compared to a desktop.

Also, laptops generally have CPUs that pull 15 watts. Whereas desktops have CPUs that can pull 95 watts or higher (sometimes up to 150). The difference is astounding.

Not necessarily. Intel is still putting out i3's with better performance than their i5 or i9 models. It's all about the specs, not the model number.

Depends on what the workload is.

CAD, for example, barring new geometry work in progress now, is single threaded.

People working on high surface count models want these things:

Big, fat, fast cache

Sustained sequential compute performance

Sustained I/O

GPU that focuses on geometry and precision. This is not generally an issue today, but can be on laptops.

Desktop machines with active cooling are where it is at.

i3/i5/i7/i9 aren’t actually different processors, they make a handful of desktop/laptop/server... variants for each generation of Core architecture CPU and differentiate down the line by binning and restrictions added at time of production. So mobile i5 might have nothing to do with desktop i5, or i7 and Xeon can be taken from the same batch.