The federal power for eminent domain that allows for 'seize the land and deploy the bulldozers the next day, deal with the court stuff later' comes from the Taking Act[1].

States have their own eminent domain powers which vary, however in general they're easy to sandbag in the courts for years, preventing the state from doing anything while the landowners argue over the money.

[1] https://en.wikipedia.org/wiki/Federal_Declaration_of_Taking_...

It's complicated, but my understanding is that the government can take things, as long as it actually has plans to use the stuff it takes to help the public. E.g. building dams, highways, etc are the common use cases.

Importantly those highways don't need to be free to use. The government can take land and build a for-profit railway, in conjunction with a private corporation if they want, as long as it was to benefit the public.

It was found in 2005 that they can take your property and give it to another private party simply by making the claim that its for the public good. This has been used to take homes from people so they can be bulldozed and replaced with new construction, the idea being that the larger tax base is for the public good. This makes the power in effect limitless as you can claim almost anything is for the public good.

A major problem I have with this is appraisal prices often don't jive with reality. For example in San Fransisco houses almost always sell above asking, if the government were to step in and give the land away to a private person at asking they would be in effect getting a discount. Similarly my own home has had a lot of work put into it that doesn't effect it's appraisal price meaningfully, if forced to sell at that price I'd lose money.

https://en.m.wikipedia.org/wiki/Kelo_v._City_of_New_London

The Supreme Court recently ruled that this is pretty general. Specifically they ruled that Chicago could take land (with payment) in order to give it to a chocolate factory that wanted to expand:

https://www.bloomberg.com/news/articles/2021-07-02/supreme-c...