NAS OS location?

Hi,
I found your website completely by chance and started to read as much as I can :-)
I am impressed, big thank you !
I have a small question but first I need to say that I am a real average person with not in depth knowledge in NASes.
A few years ago I managed to set up a NAS on my own, using OMV and various salvaged bits and pieces from old computers.
And it works(!) albeit not blazingly fast.
One thing I remember setting up my box -and it was really important for me- was that PMV required its own disk or at least (of course) its own partition.
Having the OS completely apart from my two mirrored data disks on a separate disk was/is of great value for me.
So now my questions:
With all those nice Synology/Qnap/Terramaster/etc NASes, where is the respective OS installed ?
Is it installed on the data disk? Or maybe on an SD card or USB thumb drive?
I want to buy one of these nice things (I like the QNAP TS-453Be) but I really don't want the OS to share physical space with my data disks.
Many thanks for your fantastic work
Kind regards
Michael

The operating system lives on a small internal SSD. But once it is set up and configured it will also live on a RAID. You can use one of the bays for an SSD. When you do set it up, all apps and OS will be on this fast media. If you have Qnap with M.2 slots then you can install it on this 5 times faster media. This would speed up NAS responsiveness and surely apps like Plex and more.I hope this helps.