Trump has NOT vetoed the bill. He issued a veto THREAT.
As Joe would say, here's the deal. If a bill passes both houses of Congress with 2/3 majorities (or more) most presidents realize that a veto would not hold and sign it or wait ten days and just let it become law without signing it or -- if Congress is adjourned and will stay that way for ten more days, to hold onto the bill and let it die by "pocket veto." The thing is that if a bill is vetoed, it must go back to the SAME Congress for an override vote. Generally, the Congress will, in fact, vote to override the veto and the bill becomes law then. Right now, there are three huge problems with these "normal" solutions.
1. Trump is far from a normal president. His regular procedure under these circumstances would be to veto the bill and try to strongarm the Republicans in Congress to prevent an override vote. (He wouldn't be the first to try this, but it's very, very unusual for it to work. )
2. And most important here, this Congress adjourns forever when the next Congress is sworn in on January 3. Trump could formally veto the bill and leave a few days for it to be overridden before then, but that would ruin all his fun. If he just holds onto it until the new Congress comes in, the bill is dead, dead, dead, forever dead. It can't be overridden by the new Congress. They have to start all over again and negotiate a NEW bill.
3. Oh -- and it's not just a Covid relief bill. It's the whole damn bill to fund the entire government for the current[ fiscal year. So if the idiot doesn't sign it by Monday, the government shuts down.
And if "3" happens, the only solution Congress could provide (because if he holds onto the bill, it wouldn't be technically vetoed before this Congress adjourns forever) is to extend funding at least until the new Congress takes over, when they could do the same. Oh -- but the current president has to sign that too.
More 2020 fun and games.