Hi, I would like to know if under the hood, ROS uses the default (kernel) Linux Bridge or the Open vSwitch? Or does ROS using something totally different for its Bridge-implementation?
In-kernel DSA, most likely heavily modified.
I ask, because I use Proxmox (PVE/PBS) and there, the "Linux"-network things (not OVS):
For my understanding, they behave exactly the same as the ROS counterpart even with the same nomenclature. So I wonder if its somehow the same piece of software?
Which is great btw, because you can sandbox and test things in ROS and copy the config 1:1 to PVE/PBS. Not to mention, if you (finally) understood how the Bridge works, you understand it there too.
Yes it is. It's called DSA.
Mikrotik never really cared about hiding or over-abstracting things. Most of the basic networking that's exposed in RouterOS is straight from the Linux kernel, like the firewall being 1:1 iptables, etc.
interesting thing is that MT started forcing DSA style bridge at same time when OpenWRT started to migrade swconfig to DSA
since MT is closed source, and OWRT is open ... anybody could make conclusion
It must be: ROS 7 from Outer Space.
That must be a reason why Mulder and Scully started the new case 
If the implication is that MT relies on and incorporates OpenWRT code... that may well be so. Most of the well known manufacturers probably do as well (and I think to a much larger degree than MT.)
But, Mikrotik's introduction of the DSA also coincided with the introduction of their Marvell-based switch lineup. Considering that the DSA has first class support for these chips - which is not surprising, because Marvell basically funded the development of the entire subsystem - this, at least to me, reads as a suitable alternative coincidence.