i don’t understand anything. i thought grandpa wanted to hand over everything to dojoon ...

Yes and no.
Grandpa really acknowledges Dojoon and thinks of him as a heir (imo)
But grandpa has a bit of special philosophy about inheritance. Basically, he wants to hand over everything to the most qualified heir. He would rather sell away everything than let unqualified people takes over his life business.
So, for Dojoon to take over, he has to prove himself qualified.
Now regarding this qualities, basically, it's to forcefully takeover the business, as for as I understood. Grandpa literally thinks that to prove himself, Dojoon has to steal everything from grandpa and makes it his.
And this is why, Dojoon and grandpa has this loving but merciless relationship ╮(￣▽￣)╭

That's grandpa's though love... xD
Think of the inheritance process as a championship of game of chess, ok? Gramps is the current champion and main opponent. Mc's dad gave up the game before it even started. His Aunt... went from contender to chess piece and mc is almost taking her. And from his 3 uncles the oldest thinks he's closes to winning his game against grandpa, the 2nd thinks he's doing good and competing with the oldest on who's closer to winning and both think mc is playing because grandpa is likes him and is giving him a handicap without realizing they are actually losing the game. And the 3rd one... he knows mc is good at the game and thinks that even if he doesn't win the game first he will at least win it after him instead of losing and turning into a game piece at some point like his brothers.
So... basically mc and grandpa love and respect each other and are playing brain games to test mc (and the others) and raise him into someone strong enough to have his seat and grow the company further.