Background usage people forget
Cloud photo backup, operating-system updates, app updates and laptop synchronization can consume gigabytes without an obvious foreground activity.
Disable those tasks on mobile data and review per-app permissions before travelling. Treat hotspot use as heavy use because laptops are less conservative than phones.
Choose with a buffer
Check the phone’s recent per-app mobile-data statistics, estimate a daily average and add a buffer for transport, translation and unexpected time away from Wi-Fi.
A top-up-capable plan can be more flexible than overbuying, but top-up support must be confirmed for the exact package.
Social media and video
Social feeds preload images and short videos, so passive scrolling can use far more data than messaging. Reduce video quality and disable autoplay where possible.
Long-form streaming and video calls should drive the allowance decision. Their consumption changes substantially with resolution and connection quality.