Long story, so to simplify, it goes like this:
2nd dimension Phineas meats 1st dimension Phineas, but the dimensional rifts open a rift so that a horror dimension closes in on the second dimension and turns them all into demons like the henchmaniacs. But, because of Phineas’s extraordinary brain power, he was so much more powerful than the others. But, because of candace having a firm grip on military lie, and Isabella refuseing to like him, he goes insane. He burns up his whole dimension. But after a trillion years of being alone, he’s now more insane than ever, but he also has a plan. He creates the henchmaniacs (and personaly, i think he might have made a daughter but that’s a different story) and sets forth on the one place he knows has the things he seeks: the place that 1st dimension Phineas sang to him about when he first got the idea of freedom. He not only travels through space, but time. Hence, how he knows time-baby. Flash-forward to his death, it’s assumed he takes a different form in a different time. I had many theroeys about who he could be, but the most likely candidate is 1st dimension Phineas. If you don’t believe me, how did he know that Isabella kissed him, but he got his mind erased? This was the Axolotl giving him the best second chance possible, and who would be the better candidate than a different version of himself? This also just goes to show that Isabella is (one of) the center(s) of the multiverse. He needs her to love him so that he doesn’t turn into bill again. And it just goes to show that she is the secret hero in this tragic tale!