Not to be snarky, but 'close some applications' would free up memory (and/or switch to lower-resource-requirement versions of applications - a 'big' example would be switching DEs from GNOME or KDE to XFCE or Blackbox). I know I've seen some applications (web browsers seem to be the worst, but I've observed this with office suites and some games too) that can take up quite a bit of memory (into the realm of a few GiB) even with 'nothing open' due to their own inner workings. Closing and restarting the browser/app and/or clearing its cache/storage can sometimes help there, but perhaps this doesn't apply to your situation (I'm admittedly guessing at your use-case here).
AFAIK - Dropping the cache doesn't really 'free' memory, from what I understand the OS can still send applications into that memory, but it otherwise uses it for caching (rather than let it sit empty/untouched) to improve performance. It shouldn't get in the way of an application needing memory. I found this website useful:
https://www.linuxatemyram.com/ (if you scroll down to the bottom they also link to a second page that has some examples with the cache).
