If my school only sent my transcript to UCLA, but I later decide I want to apply to UC Irvine as well, is there a way they can still see my transcript? My school won’t be sending anymore now because they made their list already and don’t want to add anything 
You do not send your transcript to any UC until they ask for it. Usually this is at the end of Senior year after you have graduated and you send to the college you plan on attending. Some UC’s may ask for Mid-year transcript if you are a borderline applicant and want more information to help with their decision.
@Gumbymom then how do they know my grades?
You self report your grades. It is important to have your transcript in front of you when you input your grades into the application. Any discrepancies noted from the transcript vs the application could be a reason to rescind your acceptance later on. All information on your application is done by the honor system. The only official documents you need to send are the ACT/SAT scores directly from ACT and College Board.
For UC application, is that sending only one SAT and ACT official score report is enough and no need to send to individual UC separately?
If you send all ACT and/or SAT scores to one UC, they will be shared with the other UC’s.
From the UC Admissions website:
Report ACT and/or SAT scores on your admissions application, then request that an official copy of the scores be sent to us from the testing agency. You can have your official score report sent to one UC campus, and all campuses you apply to will receive it.
In the College Board’s Score Choice module, ensure that all scores are sent to UC. We require all scores and will use the highest scores from a single administration.
For the ACT with Writing test, we will focus on the highest combined score from the same test administration.
For the SAT Reasoning Test, we will focus on the highest total score from a single test date.