No, they can’t stop you for no reason.
No, they can’t search you without cause or a warrant.
https://www.aclu.org/other/constitution-100-mile-border-zone
Right, except not being white or having an accent is a reason enough to be stopped.
https://www.cnn.com/2020/02/05/politics/us-citizen-dies-in-c...
And odd example to pick since it seems like he was a suspect in smuggling? That alone is enough to detain anyone.
No, they can’t stop you for no reason.
No, they can’t search you without cause or a warrant.
https://www.aclu.org/other/constitution-100-mile-border-zone