Most of the privately owned land in the United States has a chain of title that traces back to what is called a patent (a fancy name for a government issued deed) from the United States government. In the Midwest, the Northwest Ordinance provided a framework for many of the patents, and in the West, several Homestead Acts provided the land (the exact homestead act involved matters quite a bit because some homestead acts granted settlers only surface rights, reserving mineral rights to the United States, while others granted both surface and mineral rights). There are also numerous isolated patents authorized piecemeal by Congress over the years.
Some land in the United States was recognized as owned by an Indian tribe by treaty and sometimes subsequently to a patent from an Indian tribe to a private owner.
On the East Coast and the Pacific Northwest, a fair amount of land is traceable instead to a patent from a government granted a British or Dutch colonial charter, some going back to the 1600s.
Land in Florida is traceable to multiple sovereigns as it changes control several times in early North American history (almost all Indian land in the American Southeast was conquered in war by the United States or predecessor colonies or was ceded in treaties resolving those wars generally to the detriment of the Indians). The ultimate roots of land title in Florida is arguably the most complex in the nation.
There is some land in the American Southwest that derived from Mexican land grants, some going back to the 1500s.
There is some land in Louisiana and along the Mississippi River that has a root of title in French land grants.
A great deal of land ownership in Hawaii traces back to prehistoric times in the Kingdom of Hawaii and its predecessor states. It is home to the oldest continuous periods of land ownership recognized as such in the Untied States.
Most land in Alaska is government owned, tribally owned, or traces back to a United States patent, but some of it can be traced back to Russian land grants.
There are a couple of other important real estate rights.
Rights to use water in the arid West arose from people gainfully using a certain amount of water at a certain location at a legally established date, and within each watershed, water rights have priority based upon the year in which they were established. Low priority water rights are worthless when there is little precipitation and only have value in wet years.
Mineral rights are leased from mineral interest owners, sometimes owners of the surface rights or former owners of the surface rights who severed the mineral rights from the surface rights, and sometimes from the United States government (or less often, state and local governments) that retained those rights. The process of acquiring rights in minerals that were reserved by the United States is called "staking a claim" and the process is similar to the process under the Homestead Acts, except that the Homestead Acts are mostly spent and no longer in force, while there are many places where it is still possible to stake a claim to minerals.
A couple of notable statutes also did the reverse, establishing public roads in places where there was historic use of those roads, even if it would have otherwise been subject to a land grant. Private land can also be ceded to public roads by prescription (i.e. use without permission by the general public for an extended period of time as set forth by statute).
There are special rules, that are not uniform in the United States, regarding who gets land that comes into being as a result of a changing coastline. Usually, it is either the adjacent land owner or the government.
Sovereign title is acquired either by purchase from another sovereign, including Indian tribes (usually subject to private land grants made by prior sovereigns), or by conquest, sometimes recognized by a treaty with the former sovereign and sometimes not. The United States acquired sovereign title to more land by purchase from other sovereigns than any other country on Earth. But, generally speaking, colonial powers did not recognize pre-Columbian grants of private land title, even in the parts of the Americas where such a thing was recognized (e.g. the Aztec Empire). Few North American Indian tribes recognized permanent private land ownership as a concept at all.
The allocation of sovereign title in the New World between Spain and Portugal was made by treaty in 1494 CE.
One cannot obtain rights in real estate from a sovereign by adverse possession or prescription in the manner that one can from a private land owner, except when expressly authorized by a statute passed by the land owning sovereign.