iPhone offloads apps I’m currently using

I’ve been traveling the last couple of weeks, and the apps that I’m using the most are Notes (I write down everything) and iBooks (were I have Lonely Planet and some books I’m reading).

Both were offloaded!!!!!! I was in Botswana and WiFi was not easy to get.. As a result I could not use them for a couple of days.

Afterwards, my Gmail and Outlook also got offloaded.

Yesterday, DJI app from the drone I’m using daily, also offloaded.

Apps like Mobike, Uber Eats, Google Ads and other work related that I haven’t used in weeks haven’t been offloaded.

I don’t understand how this works.