macOS is built to use as much RAM as it can, intelligently. I would not concern myself with seeing how much RAM is free at all times, just use the machine and if you notice slowdowns, check and see how much swap you are using or if your memory pressure is constantly yellow or red.
As you can see, Safari has cached each site you were on but have left, so if you quit Safari and re-open it, you will free up at least a couple GB that way, but it will also automatically do that if your available memory gets low