I’m wondering too. I remember when I was in HS, one of my teachers said that they could easily spot if anyone was cheating with their calculators because almost all of the students that were not cheating would be focused on reading and writing by hand most of the time, and only occasionally reach for their calculator to calculate something with it. If someone was paying an abnormal amount of attention to their calculator, it could indicate that they had for example put notes on the calculator, or had some extra programs that had features outside of the allowed ones. So then the teacher might sneak up behind the student and have a glance at what they were doing on the calculator.

Also, to try and combat those kinds of things my school always had us show them the list of programs on our TI-84 Plus calculators before a test started, and we had to delete anything outside of what was allowed. That being said I think the calculator can be modded with custom things that could be hidden. So probably the best way of preventing people from cheating was by paying attention to who was using the calculator too much.

I was majoring in CS in one collage and doing Business Management in another collage. Both had class that required us to memorise tons of lexical knowledge. For example I had two semesters of Corporate Finances. Lot of formulas (100+) had to be remembered to calculate different KPIs a corporation. I had all these formulas in a notes uploaded to the calculator. If a test questions required one to use, I opened up my notes, looked up the formula and used it. TI-89 was able to use variables, solve equations for different variables and convert units. Using the right shortcuts I just plugged in the numbers. Of course I understood how to apply these formulas, I just didn't have the patience to memorise them. Another class was Psychology. Had to memorise a complete book. No way I would do that. Instead collected notes from friends, organised them, digitised them, created well structured Table of Contents (all took about two days), uploaded to the calculator and used it during tests. You might ask why it was allowed to use a calculator during a Psychology final test? Answer is easy. At those times no teacher could image that notes could be stored in a calculator. TI-89 was just released, hardy available in the EU. So if they saw me playing with the calculator they thought probably the kiddo doesn't know anything and just pushes buttons to spend time.