How do you get credit for AP classes taken senior year?

What do you mean by "credit?"

In order to receive college credit for an AP class, you need to take the AP test and get whatever minimum score is required by the college. This is the same for every year/class; you can take the class but not take (or not pass) the test, and so get high school credit, but not get college credit; or you can take the test without taking the class, in which case you'd get college credit, but not high school credit.

Do you mean, how will the college know what you've taken senior year? Colleges receive your senior year transcripts at the end of the year, so they can be sure that you have graduated, successfully completed your course work, and have taken level of classes similar in complexity and rigor as your prior years. And yes, colleges often do withdraw an acceptance if a student slacks off senior year.

You need to finish the course and take the AP test. Your score on the test needs to be high enough to qualify for college credit. You do not have to take the test before you apply for college. Just have the test results sent to the college you plan to attend. See that college's web site or call its admissions office for instructions on how to submit your AP test scores.

Colleges get your transcript at the end of the year.