I have chromium installed using DietPi software. I’m currently running DietPi on a microsd card. Is chromium already set up so that it writes to ram instead of disk? I don’t want to ruin my card. If not, how would I be able to do that?
It is not. And AFAIK it is not trivially possible. You need to link a regularly changing number of directories to a tmpfs. There is no single cache dir or setting to change it.
At least if Opera has not an entirely different profile setup 
.
I am open for suggestions/methods if someone knows better. In any case, it would need to be an option, not default, since many SBC would run out of RAM quickly.
Hm but according to Chromiums docs, the profile and the cache are at
~/.config/chromium
~/.cache/chromium
or at the corresponding $CHROME_CONFIG_HOME and $XDG_CONFIG_HOME diretories.
Oh looks like it is very differently setup than on Windows. Makes sense to use XDG standard dirs. I am having my first coffee just now …
So ~/.cache/chromium contains at least the common browser cache of web assets? There are also GPU and shader caches, code, system, and extension caches. But those are not that relevant in size.