Theoretically, internal transfer is easy. Because community colleges are mostly unknown schools, even if the transcripts of these schools show that your grades are excellent, they will be discounted in the eyes of the admissions office. Your A+ in ordinary community college may not be as valuable as your B+ in UC system.

In addition, a good recommendation letter is also very necessary for applying for transfer. Even poor universities with UC system will have better professors. If you can get their letters of recommendation, I believe you will have no problem moving within the system.

Finally, it is very important. The credits of universities in UC system are basically recognized. So you don't have to study in school for a few more years, in order to make up the credits that don't count.

So generally speaking, it is better to transfer within UC.