I’m just wondering if senior grades (or high school grades in general) come into play after college admissions are done? Like for certain internships in college, etc.
Thanks!
From the personal experiences of my kids, sort of sometimes they count just a little. 
They have applied for a few scholarships, organizations, internships that asked for their high school GPA (not specifically senior grades, but of course those are figured in to your high school GPA). They don’t know how much, if at all, that GPA was considered, but it was asked for. On other applications, there were no questions about HS at all. Some college students will put their HS GPA on their resumes, especially when they are freshmen and perhaps feel that their HS GPAs are recent enough to be relevant, or good enough to stand out, or the new college student doesn’t yet have a college GPA.
If your senior grades are outstanding, you may want to find ways to show them. But from your question, I suspect you don’t like your senior grades? In that case, I suspect it will be easy for you to leave them behind. Don’t put them on your resume, and even if an application asks for them, just be sure your college record will stand out. Certainly people will care more about a great college record than any HS record. Plus, you will have a great point to discuss if it ever comes up in an interview - what you learned, how you’ve matured, or whatever.
I guess one other thing to consider is that colleges can rescind your acceptance, so senior grades certainly count in the time between being accepted and beginning college. But I don’t think that’s what you were asking.
If you attempt to transfer to another college before junior level, transfer admission will generally consider high school records to be relevant. Even for transferring at junior level, some schools may still consider high school records, although you will have enough of a college record that the college record will be the most important part of the application.
Some elitist employers want to know your SAT scores from high school days even when you are about to graduate college or after you have graduated college.
Generally speaking, senior grades do not matter post high school. Are there exceptions, yes? Since you don’t know what the exceptions will be, why not just make it a nonissue by doing the best you can as a senior.
As Jugulator20 said, it probably won’t come up, but you never know. My daughter just finished up law school applications in January. One of the 9 she applied to asked for her high school gpa (Notre Dame? One of the UCs?). She didn’t remember exactly what her final gpa was and had to get it from the registrar of her high school.