No need for a warrant. So pretty much anyone or anything that is in line to cross a border is automatically fair game. I'm sure there's case law on this. For example suspicionless DUI checkpoints have been ruled legal because the supreme court views it as a minimum intrusion and for the greater good. So wouldn't surprise me if similar grounds are used in the border case.
However some state courts have said DUI checkpoints are illegal under state constitutions. For example in Texas, however Texas has border patrol checkpoints. Never knew of that from being from the midwest, but I follow some full time RV vloggers on YouTube and learned about them. It surprised me, since not leaving or entering the country so never expected that traveling within your own country. Within 100 miles of the Mexican border, both permanent and temporary. Then recently in the last several years they do them now for the northern Canadian border too. Maine, New Hampshire, Vermont but temporary for a few days a year. Some people live and work near them so they have to go through a checkpoint every single day, and traffic backups too. So annoying for the local residents, and also might discourage some tourism too, hurting their local small businesses like restaurants.
They are supposed to be checking for immigration status, but even looking for other things such as drugs. Even though drug checkpoints in themselves were ruled to not be legal. Since weed is legal in California, federally it's still illegal. So if you drove across California with weed, you could still get in trouble. There's some videos of people challenging them and refuse to answer if they are a citizen. Even border agents claim their dogs can smell if someone is an illegal or not too. So I guess illegals and citizens suddenly have a different scent.
They also board buses and trains such as Greyhound and Amtrak going seat to seat asking people of their status. The ACLU has been asking Greyhound to stop allowing them to do this since the buses are considered private property, but there's also arguments since they are commercial they are subject to different laws. https://youtu.be/0SxHkFZV4fw here's an example of one before getting on a bus or train at the station. here's one in Florida https://youtu.be/U62XSk8uQtU
Phoenix on a metro bus. https://youtu.be/XgQVrBGlqx8
Spokane, WA City Council had a issue with it too. https://youtu.be/xHKk8oZLA1s and https://youtu.be/GjbLJqvZVDo - They get on the bus and go seat to seat asking asking what country they are a citizen of and where they are going... You don't have to answer but cases where they have taken people off the bus and can make you miss it. Pretty sad traveling within the same country or even city. I think they should keep border security at the border and secure the border itself.