Where are all the MyBB codes written (ex. {$membercount} etc.)?
There isn't a list of them as there's hundreds, they're just PHP variables defined all over the code wherever they need to be set to a template. Do you need to know if anything in particular can be used in a template?
OK, then how would i know that there's this specific code for this specific thing?
You would need to look in the code to see what variables have been defined; compiling a list would be near enough impossible unfortunately.
OK
Is there something specific you're looking to use in a template? I can have a look and see if it's available or not.