The express edition has < 1GB of RAM, and < 11GB of user data
< 1GB of RAM --> is this a combination of SGA + pga aggregate target etc?
< 11GB of user data --> does it include temp, undo, system, sysaux tablespace or the temp/undo/system/sysaux has not such limitation?
>> combination of SGA + pga ?
Yes. Total memory footprint for the instance can not exceed 1GiB, sga + pga is pretty much the definition of the instance memory size.
>> include temp, undo, system, sysaux ?
Those tablespaces should not count towards the user database 11G limit. System and sysaux are for the instance catalog and resource monitoring/tuning utilities, don't put stuff there. Just don't. Using system or sys schemas for storing user data is just plain bad form.