iPhone users get one genuine advantage in application season: the apps already on the phone are good enough. Before you download anything, look at what you own.
Reminders, used properly, is a milestone tracking app for college applications. Deadlines as dated reminders. Document checklists per school. It syncs, it nags, it costs nothing. Unsexy and undefeated.
Google Sheets on iOS is still the most reliable free applicant progress tracker app. Same logic as anywhere: one row per school, columns for everything that matters. Not glamorous. Has never lost anyone's data, which is more than I can say for some dedicated apps.
Trello and Notion both have solid iOS apps. Trello for the board view of your school list. Notion if you want a full profile building roadmap app with linked databases for tests, activities, and essays. Both sync to desktop, which matters when you are writing essays on a laptop and checking deadlines on the phone.
The App Store also has dedicated college planning apps for high schoolers. Some are fine. My skepticism is about maintenance: these apps live and die on their databases staying current, and small teams abandon apps. Check the last update date before you hand your whole application season to one. An app untouched for two years is a museum piece, not a tool.
One more honest point. The best tracking app is the one you actually open. A perfect Notion setup you ignore is worse than a notes app you check every morning. Pick the simplest tool you will use daily, then actually use it daily.
Whatever you choose, the milestones matter more than the tool. Test dates, waiver windows, counselor meetings, submission deadlines. Miss one and the app did not fail. The plan did.
Check fee and waiver milestones against the database at applicationexperts.fyi. And to build the full two-year milestone plan, get a 1-2 year profile roadmap.