Look at table from link above. From my experience for infrastructure and fullstack monitoring by defult DDU's are not consumed. Event some basic plugins most of the time are fine with embedded quota. But when I enabled for example plugin for haproxy which had 60 dimensions for each metrics I've started to have consumption. Level of consumption is available for you in dynatrace settings so even if you enable extra plugins (custom or embedded one) you will always have option to check consumption there and decide if you want to have them or not. DDUS are used as well for log metrics (if you will configure some).
DEM's are consumed only when you will enable real user monitoring and Synthetic tests: