Windows has been doing prefetching and caching all the way from 7 but even more so since 10. Also since 10 it does memory compression too.

RAM usage is RAM usage, and besides the allocation still being awful and you probably having less RAM available in a heavy task, this means substantial power consumption, that costs money.

Unused RAM is wasted RAM. Even if it's just the OS keeping apps on memory for faster launches. If you do need heavy RAM for a task your OS is clever enough to reshuffle things.

Used RAM does use more electricity but that is so neglible it's a non-issue and no argument.

Of my 32 GB at least 26 GB are constantly in use.