I’m a little lost in the manga as a whole
Okay, brief summary of the manga:
Once upon a time, there was a fantasy world. It had every fantasy cliche: sword and sorcery, heroes and demon lords, and epic battles. In the final war, heroes defeated the demon lords, but the battle claimed the lives of all the gods, and untold number of other people.
Now it's all a distant past. The world has developed into a technological world not unlike ours (except with more magitech), until another cliche happened: scary monsters called Void appeared from nowhere, bringing apocalypse. The only way to fight them, is with teenagers armed with greater-than-magic powers called "holy swords" (because stereotypically, the powers manifest as an awesome melee weapon).
People of this world did the only reasonable thing in these circumstances: built mobile fortress-schools dedicated to fighting the Voids. Because everybody knows: teenagers are much stronger when they are schoolgirls.
Then comes our MC. He's got the best parts of it all: he's been a hero and a demon lord, a swordsman and a magic user, he has slain gods and made friends with fantasy people, and finally sealed himself away so that he could take part in the modern events too.
He made a mistake with his sealing spells, and has awoken as a cute little boy instead of a badass lich he used to be. This worked in his favor, as he quickly got himself a harem of teenage schoolgirls (because all teenage girls are shotacons). And a position as a student in the magic fortress-school, which he uses as a cover for his demon lord activities.
What demon lord activities? Well, he plans to take over the world and present it to the goddess he used to serve. Because he believes she will be reincarnated about now. We're given hints that the female lead might be her.
In the meantime, MC fights the voids, because he can't take over the world if the Voids destroy it first. And tries to gather information about both humanity and the voids, because all wars and conquests start with information gathering. It's fun seeing the MC think "I want a ship like that to be a flagship of my fleet. It's many times larger than the ship of doom I used to have".