True, galaxies can pass through each other with very little star-to-star interaction (stars are so far apart from each other compared to their sizes--several light-years vs several light-seconds). There are numerous cases of galaxies in the act of colliding now under observation. These events take millions of years to play out and they are carefully studied.

But though stars may not interact much, the large gas and dust clouds in the spiral arms DO interact, colliding, giving rise to shock fronts thousands of light years across, pressure waves that precipitate bursts of star formation along their edges, like foam bubbles on a whitecap at sea. Most stars formed this way are run-of-the-mill, dim and long-lived dwarfs and main sequence stars that quickly settle down to long, uneventful lives, but a tiny minority are massive, blue supergiants that put out extravagant luminosity until they burn out and go supernova after just a few million years.

On a galactic time scale, these collisions are violent and spectacular events, but to real-time human eyes, its about as exciting as watching grass grow or paint dry. We have to speed up these proceedings mentally to grasp just how incredibly violent and disruptive they are. Galactic studies are like forestry or glaciology. There's a lot going on, but it is spread out over unimaginable deep time.

I would expect the galactic nucleus would be generously populated with stellar black holes, the end result of big supernova events. This is the oldest part of the galaxy, and the most populated part. However, I wasn't aware that there were multiple supermassive black holes (several million solar masses)lurking in galactic nuclei, evidence of previous encounters. (Any references?). There is at least one supermassive black hole present in most galactic nuclei, but I wasn't aware that there were "several". We can see truly catastrophic long term events at multi-megaparsec distances which might be evidence of the mergers of such objects (AKA "active galactic nuclei", and there are occasional transient events even more powerful in nature we can see all the way out to the edge of the observable universe. Let's hope nothing like this ever happens in our own galaxy.