The operating system (DSM) is only installed on the hard disks by you during the first start-up. These are empty even if you receive such a set (Synology + HD). The latest and most up-to-date version of DSM is installed without intervention in the setup wizard.

https://kb.synology.com/de-de...
https://www.youtube.com/watch

I can't answer this 100% myself, it also depends on when the device was produced, but considering that it came out in 2018, I would guess DSM Release 6. However, you can immediately update to the new 7.1 DSM Release version when you set it up. However, I would recommend buying a DS418, because with a 2 bay NAS you will have big disadvantages later on, if the NAS has to be expanded, then two disks have to be replaced (you can also see this problem on the usual platforms where 2 bay NAS are offered in large numbers). I would recommend the DS418, it is a better solution also in terms of cooling, because it has 2 fans at the back (redundant and not audible in silent mode), you can also start with 2 x 3TB disks, but then upgrade to Synology Hybrid RAID with 3 disks or even to 4 without having to replace already purchased disks.

My tip would be a DS418 and two WD disks, it's a bit more expensive, but in the long run it's worth the extra cost:

Synology DS418

Since the surcharge for a WD RED Plus 3TB to 4TB is only 7 francs, I would use two WD RED 4TB right away:

WD Red Plus (4 TB, 3.5", CMR)