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.
Personally I use simple tab group that allow you to separate tabs into groups that you can open in different windows. It’s extremely useful but it means sometimes if you switch between multiple tab groups you might have a lot of tabs open, but using this would allow you to majorly mitigate that problem.