When using emuMMC, the space on your SD card is split into 2 parts. The allocated space is for your emuMMC partition and the unallocated space is the remaining space that wasn't used for the emuMMC partition. Since emuMMC is meant to be a clone of sysMMC, that also means we have to clone the internal memory of the sysMMC. There are 2 ways of achieving this; either file or partition-based emuMMC. File-based emuMMC has the internal memory of the emuMMC be treated as a singular file whereas partition-based emuMMC allocates part of your SD card's storage as a partition to be treated as internal memory. Since this space is already taken up by the partition, installing content to the internal memory of the emuMMC won't take up more space. And you can always check how much internal memory is left by booting into emuMMC and going into Data Management.
Yes, you can just turn the console off to use it like normal. As for the best guide, I always recommend
this one as it was not only the first guide to ever be released but it has also been regularly updated as the scene progressed.