What provision of the Constitution allows the Federal Government to "own" land in any state? Also, what provision makes the BLM legal?

It is actually the agreement between the U.S. and Nevada when Nevada became a state that gave the U.S. land and is in the Nevada Constitution: "That the people inhabiting said territory do agree and declare, that they forever disclaim all right and title to the unappropriated public lands lying within said territory, and that the same shall be and remain at the sole and entire disposition of the United States"

The public land in all western states became property of the U.S. when they became states, except Texas. Because Texas was its own country and not a territory of the U.S. when it joined the union, it maintained ownership of all its public land.

The BLM was created by Congress and is neither authorized or prohibited by the Constitution.

The fact that it owned it previously.

The government has always owned property and the 5th Amendment recognizes this by specifically limiting it's acquisition of property. The Founders of America would have laughed at you for implying that they need to spell this out. The fact that they paid for the revolution by selling land they owned in the northwest territory makes you an absurd buffoon.

And it is equally absurd to pretend that they didn't know you need bureaucrats to operate the government, as they used such bureaucrats at the time. The BLM is no different from the Customs Service.

Amateur historians and amateur political science students just waste people's time. Thinking they found typos which prove their lunacies.

"The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States...."

ARTICLE IV, SECTION 3, CLAUSE 2

Add: When a state enters the union the land belongs to the state AND to the United States. Basically, the land is held by the state in trust for the US. However, the Supremacy Clause of the Constitution says that where state law and federal law conflict, federal law trumps. Article IV which I quoted above gives the federal government the power to make "needful" rules and regulations concerning US lands. If Congress exercises this authority (as it did in Bundy's case), then any state law which conflicts with the Congressional authority is void.

The only argument is whether the designation of federally protected land was "needful" in this case.

If you really want to know the answer then read the rulings of the THREE court cases that the rancher filed and LOST.