How do I free up storage occupied by applications on iPhone?

Delete some applications, especially ones you don't need or use. In Settings > General > iPhone Storage - you can also turn on Offload Unused Apps, which will take them off your phone, but keep their data. When you remove apps individually, by pressing on the app until the screen jiggles, you can tap remove the app as opposed to delete the app, which will also keep the app data for future redownload.

And find the actual applications top of the list, I bet they are Whatsapp, Insta, Snap etc.

Go into the app and delete all those cute videos, they are probably in your Photos anyway.