Short answer: cleaning up storage on an iPhone 18 has no official one-tap button. The method that works is to open Settings → General → iPhone Storage, look at the usage list, and work down it from the biggest entry: photos and video, chat app caches and attachments, offline downloads in video and music apps, and apps you installed and forgot.
When you are done, keep headroom — iOS needs free space as scratch space, and below roughly 10% free the system has to make room before it can do anything, which shows up as failed installs and a phone that feels slower and warmer.
Look at the list before you delete anything
Open Settings → General → iPhone Storage and you get two things: a summary at the top (used and available) and a list of apps underneath. That list is your to-do list. Working down it beats deleting at random, because it tells you where the space actually went rather than where you assume it went.
Two things are worth checking while you are there: how much is actually free (more on that below), and whether System Data has grown abnormally large.
Work down these four categories
① Photos and video (almost always the biggest). The thorough approach is export then delete: copy the originals to a computer or the cloud, confirm the backup is complete, then remove the local copies. If you use iCloud Photos, turn on Optimize iPhone Storage so full-resolution originals stay in the cloud while the phone keeps smaller optimised versions.
② Chat app caches and attachments. For apps like WhatsApp, WeChat or Telegram, most of the weight is images, video and files inside conversations. Their in-app storage screens let you clear attachments per chat. If you plan to delete and reinstall an app to dump its cache, confirm the chat history is backed up or migrated first, or the messages disappear with it.
③ Offline downloads. Video, music and podcast apps quietly hold a lot of downloaded media. This is the cheapest category to clear, because everything in it can be downloaded again.
④ Apps you installed and forgot. Use Offload Unused App rather than delete: it removes the app itself but keeps its documents and data, so a tap brings it back exactly as it was.
How much headroom to keep
Storage is not a "use the last megabyte" resource. iOS needs free space for virtual memory and temporary files, and a system update has to extract an installer locally first. The practical rule is to keep 10–15% free, and free up extra before a system update. The per-capacity recommendations and the reasoning are in How much storage headroom do you need?, and Why does free storage affect speed? explains why a nearly full phone feels slow across the board.
Two things specific to a new phone
- A migration brings the old phone's clutter with it. Caches, duplicate photos and games you finished long ago all arrive on the new device. Clearing them while the phone is still mostly empty is the cheapest this job will ever be.
- Do not do anything drastic during the settling period. In the first days the system is rebuilding indexes, analysing photos and syncing iCloud; an erase-and-reinstall now just restarts that work. Delete a few of the biggest items off the list, and save the heavy options until settling is over.
One more note: System Data has no official one-tap clear, and it normally reclaims itself. If it looks abnormal — past 40 GB and still climbing — the reasoning is in What is Other storage?.