The feature is called Tab Unloading, and weirdly enough they made it not easy to access despite its usefulness.
You basically have to type about:unloads in the address bar and hit enter. If you then click on “Unload”, it will put the least used tabs to sleep. If you keep clicking that button until it’s greyed out, you’ll have unloaded all your tabs from memory.
This feature is handy if you want to temporarily switch to something that is memory hungry without having to close your 100 tabs.
From what I understand it basically just saves the minimal state possible (URL, form inputs), which is lighter than keeping all the rendering details in memory, so maybe that minimal representation still stays in RAM as its footprint would be negligible.