The
I2 does have separate file system memories to use your phrase. Also it confusingly uses the RAW and DNG terms to refer to both video and photos. RAW DNG generally refers to photographs and CinemaDNG refers to video RAW. Just to confuse you even more, the CinemaDNG format effectively comprises a series of RAW DNG still photographs that are take at 24-60fps and which are then combined in the video editor to make a video. Theoretically, you could edit each and every one of those photos in Lightroom or Photoshop CameraRAW before turning them into your video... would take you a while to do though.
Your
I2 always requires the use of a MicroSD card, and that stores photos (jpeg and DNG Raw) and video (.mov/.mp4 in various compressed H264 and H265 sizes and formats). It can also use an
optional CineSSD card (also refered to as simpy an SSD card). The
CineSSD is used for the higher spec/bandwidth video modes, recording the less compressed (ie higher quality) ProRES and CinemaDNG formats, and also
some RAW DNG photos in certain circumstances.
The
CineSSD card is more restircted in what it will record - it won't record the lower quality compressed H264/H265 mov/mp4 videos, only the high quality ones (Prores/CinemaDNG). It also cant be used for taking jpeg photos, only RAW DNG photos in the high speed burst mode (14+fps). It also
must be used with the MicroSD card installed, and stops recording when the microSD card is full. (So basically what I'm saying is that the
CineSSD isn't a replacement for the microSD card.)
Think of the
I2 as being like two separate video recorders working from the same aerial - one (microSD) records lower quality (but more quickly/readily useable) video, while the other (
CineSSD) can simultaneously record a higher quality (but less readily useable) video. Kinda like VHS and Betamax (if you're the right age

) or DVD and BlueRay - both do the same thing, but at different quality and need different playback equipment.
Adobe Premiere should be able to cope with all the formats, but it'll take vastly different amounts of time to work on each video type. The stuff coming off the
CineSSD taking longer to download, import and edit than the stuff off the MicroSD. The smaller lesser quality formats should be quicker to use... but the fly in the ointment at the moment tends to be the H265 version of the video, it's relatively new and, unlike H264, a lot of computers don't have the hardware decoding built in to their processors/video cards - that means it takes them a lot longer to display it as the computer's processor needs to chew its way thru them while it deals with the rest if your computer too.
If you decide you want to play with the higher quality format CinemaDNG stuff, Davinci Resolve seems to be able to process the CinemaDNG stuff far faster than Premiere can at the minute, so it's worth taking a look at.