The closest equivalent is AP exams, which I'd say are somewhere between GCSEs and A levels. Students study the subject for a year then take the exam. Usually they can start taking them in either Sophomore or Junior year. They can take a lot - I've heard of kids who have 15.
There's the IB which I don't know anything about.
There is also the SAT or ACT test which are national tests used by many colleges.
There are also SAT subject tests, which again are national tests but at subject level eg SAT Math, SAT Physics.
Then there's your GPA, which tracks your ongoing scores throughout your time at high school. The subjects are tested through a mix of ongoing assessment and exams.
Colleges will look at some or all of the above. They will take into account how well a student did compared to the average student at their school - so if you're at a high achieving school they'll expect to see better grades.
They also look at other things such as volunteering, leadership, sports achievements, club membership, civics, etc.
In some ways I think it's less stressful than the UK school system because there's so much more chance to do well and prove yourself, without so much being based on specific pieces of coursework or exams.
OTOH every piece of work you do potentially affects your grades, so if you want to achieve good results then you need to be a consistently hard worker, while also potentially being very involved in a lot of sports, clubs, etc.
Those are just my opinions - others may have different thoughts or experiences.