Mobile data, Wi-Fi and airplane mode

In short: On the go your phone uses mobile data (4G/5G) from your plan, at home Wi-Fi. Airplane mode switches off all radio connections.

In more detail: Mobile data is usually limited (data volume). Videos and downloads use a lot. Over Wi-Fi you use no volume.

In Depth

Checking data usage: Settings → network → data usage.
Data saver: Limits background data.
Hotspot: Your phone shares its connection with a laptop or tablet (uses volume).
Roaming: Abroad costs may arise. EU roaming is usually included, check elsewhere.
Airplane mode: For safety on a plane, or to save power quickly. Wi-Fi can be switched on again separately afterwards.
Bluetooth: Short-range radio for headphones and speakers; off when not in use.