MacOS uses virtual-memory / swap a lot
First - yes, the OS in general will use RAM before swap. There's no loss of computational power involved in that.

You seem to have jumped to the conclusion that macOS is using swap, when it could as well be using free memory based on the htop screenshot. You cannot make that conclusion from a simple sample of the memory usage.

In reality, you could have been running programs that were using a lot of RAM forcing macOS to swap memory to disk. Then you quit some of those programs (or they deallocated a chunk of RAM). Then if you take a htop sample now, you'll see that RAM usage is low (lots of free RAM), but you're also using lots of swap. 

But that is not a problem - in fact it is intended to be this way!
It wouldn't be wise for macOS to start swapping in those pages that have been placed on disk. It will do this when they're required (i.e. used by programs), but until that happens, it could be that those pages would never be used - and thus time spent swapping them in will be lost.

You write that in comparison to your Ubuntu PC you observe a different behaviour. You should know that you can actually change that behaviour. On Linux you can use sysctl to set the vm.swappiness variable. It will change how aggressive the OS is in swapping things out to disk.

You also ask why your macOS system does not use fixed size swap as Ubuntu does. In fact, Ubuntu also supports dynamic sized swap using the "swapspace" command. You can set minimum and maximum sizes, and it will dynamically add/remove swap space as needs change.