will colleges look at my senior gpa?

You'll need to apply to most colleges by January 1, so that will be at most half of a year of senior year grades. So yes: junior, sophomore and freshman year GPAs are more important, because they represent entire years, not half of a year. 

That said, it can't ever hurt to show improvement. But determine "match" colleges based on your junior year cGPA, not on your senior year half-term cGPA.

College admissions will look at every grade on the transcript they receive.   The reason some people incorrectly think that senior year grades aren't considered is that if a student applies in October, there are no senior grades on the transcript that can be taken into consideration.  Often students will send a partial transcript that includes "grades to date."

Even after acceptance, students are required to send their final senior year transcript as proof they graduated.

That's true because you should be applying for most colleges at the beginning of your senior year in which case your Senior grades would not be available.   You can of course delay applying but most competitive colleges do not accept most late applications for the standard Fall start.   If you are determined to get into a competitive college that you do not currently qualify for you can go to something like a community college get your GPA up high and apply as a transfer student once you have completed the required coursework.  Going to a cheaper college with intend to transfer is also a great way to save $1000 in tuition costs.  At the end of the day they will only look where you got your degree from, not how\when you got in.

Since most new students apply to college early in their senior year of high school, senior grades are simply not available.  After students graduate from high school, the college they will be attending will get a final transcript.  The college uses that transcript to verify the student graduated, and to ensure the student maintained grades which are satisfactory to the college. 

Yes, colleges will look at your senior GPA. In fact, many colleges consider senior year of high school to be the most important year. You can really help your resume by trying out for your school's basketball team this season :)

On what basis did your friend tell you this?
When you apply to university, you submit your latest transcript-- which will probably have your freshman-sophomore-junior year grades; but no admissions offer is valid until the university sees your senior transcript.

They will see whatever is on your transcript at the time they receive it.

They still need your transcript. Even if they don’t look at the grade 12 marks

Unfortunately, yes. Your senior year GPA is only sent to colleges if they request it. Otherwise, they only see freshmen through junior year