He's their president. If he wants to build a bridge, he can build a bridge
He's not a king, but the Executive branch of our government. He can ask that a bridge be built, but one cannot be built by fiat-unless he has the personal resources to fund such an endeavor.
It's the legislative branch-the USian Congress (house of reps and senate) that control the purse strings. So, he needs to ask Congress to fund it.
The Dept of Transportation, which is an agency within the Executive Branch, does have certain funds (given to them by Congress) to cover emergency needs; however, those funds are capped well below the cost of this bridge rebuild.
PAK is right, the shipping company-ie, its insurer-is going to be liable for the lion's share. However, this will be litigated, so something needs to be done while it's being litigated. And whatever is done is going to have to go through Congress and/or the State of Maryland.
Biden was wrong and reckless in saying feds would pay for it all. He said it because it's election season. Well, that and he is a sociopath and congenital liar.
@Bobko
You pretty much nailed it.