i'll chime in here for southern section and central coast section
Central Coast Section:
Week 1-2: League trials and finals. Each league is given heavy leeway to figure out how they want to hold these, and they vary greatly across the section. SCVAL, one of the larger leagues, splits into two, and then holds prelims on Tuesday and finals on Thursday. The top 9 times in each event across both divisions of the league progresses to League finals the following Saturday. Some smaller leagues will hold an overall league prelims on week 1, and then overall finals on week 2.
Week 3: Section trials: Each league gets a certain number of automatic entrants to section trials, each based on their size. However, there are also at-large times people in each league can hit to advance anyway.
Week 4: Section finals: Top 9 from the prior week advance to section finals.
Week 5: State meet: Friday/Saturday. Top 3 plus at-large qualifiers advance to the state meet. Prelims on Friday. Top 9 advance to finals. Prelims on Friday is oftentimes more difficult than finals the next day, especially in events like the 800. I think it took a 1:51 to make the 12-man final last year.
Southern Section:
Southern Section, as the largest section, has an unreasonably convoluted system to filter out the best. It's hard as hell to advance if you are in southern section.
Week 1: League prelims and finals. Some leagues have prelims, some don't it really depends.
Week 2: Southern section divisional prelims: each school is divided into whatever division it belongs in, where they then participate in a prelims (and finals) round against other schools of the same division. Prelims for each division are held at differing locations depending on division.
Week 3: This week is the real test for southern section runners. The top 9 in each event for each division from the prior week advance. In order to advance to the masters meet next week, you must place within the top 9 (or more for distance and field events) overall times from ALL divisions that day. In practice, this means that the vast majority of runners need to run all-out efforts, because you are racing against people of each division without actually, physically racing against them. Because of potential weather advantage otherwise, divisional finals are held on the same track for all divisions. Each division's race goes off one after the other.
Week 4: California's system of at-large time essentially exists solely for the southern section. Despite getting 6 automatic entrants to the state meet (in comparison to basically every other sections 3), such is the depth in the southern section that the last 3 runners in each masters meet final could likely still make state finals on a good day. All 9 runners from the masters meet final will often run under the at-large standard, in comparison to other sections, where you would be lucky to see 1-2 runners hit the at-large in an event.
Week 5: State meet. It should go without saying, but this section DOMINATES the state meet.