Let's see.......

A hapless guy was returning home after failing in his 100th job interview. On his way back he got hit by truck-kun and got isekai'd in a sword & magic world. But he wasn't the hero or an aristocrat. But he is reincarnated as a lowly commoner in this feudal medieval world.
Soon he realizes this new world is a mix of Isekai Anime & games.
He took advantage of the situation and he finally found a job i a certain newly emerging kingdom.
Thus, our protagonist began his career to conquer the isekai Multivarse, while building his harem.
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[ This is a kingdom building/army/geopolitics/civilization/wars/realistic worldbuilding novel ]

The premise of this story is in the title. Sorry but can't think any better synopsis then this, I'll fix it later. This fantasy is colossal. The ultimate crossover of fantasy world.Every single anime below are in story. Now the world's involved are,

Overlord
Re:Zero − Starting Life in Another World
Fairly tail
The Rising Of The Shield Hero
KonoSuba
Demon Slayer
That Time I Got Reincarnated As A Slime
Genshin Impact
Black clover
Is It Wrong to Try to Pick Up Girls in a Dungeon?
Goblin Slayer
Any villainess manhwa
ETC