Every school has a different process; some schools use rolling admissions, some release interview invitations and/or decisions in batches, and some release them for all applicants at once. A few schools display their general admissions timeline on their websites, but most don't. As for which applications are "read" first, unfortunately, there's really no way to know. For rolling-based admissions, I would assume that they process generally in order, albeit probably in batches, after a first round of auto-rejections for those who don't meet the minimum requirements or are missing pre-reqs, but this is pretty much impossible to answer for the majority of schools except by those who actually work and/or are very heavily involved in admissions. I imagine that there's likely a myriad of ways that they could do it and, again, it all varies by school.
In terms of individual overall admissions timelines, though, if the schools you're applying to don't publicize that information, I would recommend that you peruse older applicant threads for the specific schools that you applied to, particularly last year's (c/o 2025). While things are liable to change from year to year, this should at least tell you what should come out and approximately when.