Questions about Brau (and the ending)

It makes a little more sense in the manga that that's the very last scene (in fact, it's even after the production and editorial credits of that volume, where in other series you might find an omake), so you can imagine a time lapse between the volcanic stuff and that. There's also a few panels showing his dungeon empty and of people panicking that he escaped, but It's not explained how he did or got to Thracia though.

As for why he knows so much, I think there are a couple reasons for this. Both the anime and manga leave a lot left unexplained but we can speculate. First like said above he is likely a highly advanced robot up there with the seven, Abullah and Thracia's ultimate computer (Roosevelt), as demonstrated by his capacity to kill.

Second, advanced robots seem to have a unique telepathic-like ability to comprehend each other's emotions from a distance that goes a bit beyond Internet Wi-Fi, Uran is even capable of seeing what emotions animals and people (e.g. Dr. Tenma, that kid on the street) are feeling.

Third, is that he has been through similar experiences of hatred and the desire to kill himself and can probably relate to and comprehend Gesicht and Abullah. Some have speculated that BRAU-1589 is modeled after the "Blue Knight" from Astro Boy, who has a similar story about his robot family being killed and then becomes capable of killing humans. He is defeated when his enemy takes his own lance and throws it into his chest, just like BRAU-1589 has one in his. That makes me wonder then why he sided with Atom against the ultimate computer? The Blue Knight originally wanted to build a Robotonia free from humans, which seems more in line with the ultimate computer's goals in Pluto. I think the answer to this is more seen from the chronologically succeeding arcs of Astro Boy (Volumes 19 & 20 from Dark Horse's edition), where only Dr. Tenma can repair Astro Boy, and like a mad scientist he has become convinced Astro Boy will be able to lead a future where robots rule the world, (a theme I believe is explored at greater length in the 2003 Astro Boy anime), Pluto seems to take cues from here. When BRAU-1589 felt Astro Boy's heart, he later compared it to the President to understand he is like a human. But Roosevelt is a disappointed BRAU-1589 has not become like a god or ascended to a celestial realm (differing translations or dialogue) like him. Does BRAU-1589 believe that Atom, who outflanked the ultimate computer's calculations, is superior? Not just human, but maybe even super-human? Recall in the first episode / volume BRAU-1589 says humans could simply get rid of him by removing the spear from his chest, but were too afraid to do that, meaning he had to risk his life for this and it was big favour to do for Atom.

Lastly, there is a line that seems to have been cut in the anime, unless I missed it elsewhere. In the scene where Dr. Tenma and Abullah are discussing introducing an emotional bias into the Advanced AI (beginning of Volume 7 and episode 7) Tenma reminds Abullah that he wanted to build an AI that could rival Thracia's ultimate computer, warning him that doing so could make an enemy out of Thracia. If Tenma and Abullah know about the existence of Thracia's ultimate computer, it is possible others can know about it as well, maybe even BRAU-1589.

Brau 1589 is a high spec robot just like the top 7 but not literally like them but you have a point. I don't even know how he escaped and ended up in Thracia.