As I see it - but remember that my comparisons are often very simplified - you have a given number of interfaces on your Mikrotik, they are normally "self standing", i.e. ether1, ether2, etc.
If you have 192.168.2.42 assigned to ether1, the address is that of the single interface and only of that interface.
ether2 - that has no IP address assigned - is not reachable at L3 level.
When you assemble them into a bridge is like if you put them in a semi-transparent "box".
What you can "see" and "touch" now it is not anymore the interfaces, but the box containing them, but with some good lighting and squinting a little bit, you can see the singe interfaces inside it.
On the box there is - printed in large friendly letters - the name of the box (i.e. "bridge1"), but besides identifying this box by its name, you also need to reach it via IP.
So, you assign an IP to the box hand writiing on it (say) 192.168.2.43.
This address makes everything inside the box to be reachable at that address.
It doesn't matter if the cable is physically connected to ether1 or ether2, and without any other change, if you remove from the bridge ether1 and add to it ether3, ether3 would become functionally equivalent to what ether1 was doing before.