Do they completely disregard senior year and freshman year, and if they do, for what reason?
the UC’s have requirements for your GPA senior year or you can be rescinded. They check your final transcript during the summer. Each has its own requirements, you need to check by campus.
Freshman and Senior grades are not used in the UC GPA calculation but are reviewed to make sure you meet the a-g course requirements along with course rigor. As stated in the above post, Senior grades will be used to make sure you fulfill your provisional admission by maintaining a GPA of 3.0 weighted (UCLA/UCB 3.0 unweighted).
Also, there is just not enough time to evaluate the grades in the senior year (1st half of 12th grade) and have a final decision processed around late January to early March for all the numerous applicants who qualify for UC admission.
However, if you are on the edge or borderline, the UC admission committee may ask you to submit supplemental information which likely includes your grades or expected grades from the 1st half of the senior year (1st semester). The supplemental information request is few. You should still keep up the high grades (A average), at least for the 1st half of your senior year, just in case, you are asked to submit supplemental information. After that, you can just maintain the B average stated above.