Storage is the specification with the largest gap between cost to manufacture and price charged, and it is the one you cannot change after purchase.
What actually fills a phone
Photos and video, overwhelmingly. A minute of high-quality video can consume several hundred megabytes, and modern phones record at settings that produce very large files by default.
Apps matter less than people assume, except games, which can be enormous individually.
Offline media – downloaded films, music, podcasts – which accumulates silently.
The practical guidance
128GB is adequate if you use cloud photo backup and do not record much video. It fills faster than expected otherwise.
256GB is the sensible default for most people and the size most likely to last the life of the phone.
512GB and above makes sense if you shoot a lot of video, keep everything locally, or play large games.
Check before you buy
Look at what your current phone is using. That number, plus growth over the years you will keep the new one, is the actual answer and it beats any general recommendation.
Reducing usage
Cloud photo backup with local optimisation, which keeps smaller versions on the device.
Lower video recording settings, since most people never watch their footage at the resolution they recorded it.
Reviewing downloaded media occasionally, which is usually the largest recoverable chunk.
The thing to avoid
Buying the smallest tier to save money and running full within a year. A full phone slows down, cannot update, and cannot take photographs at the moment you want to.