How do I delete data and documents from apps on my phone?

Then...yes, IPhone storage is a good place to start...

---> https://support.apple.com/en-gb/HT201656#recommendations

"In the Storage section of Settings, your device might offer recommendations for optimising your storage. Tap Show All to see all the recommendations for your device.

Read the description of each recommendation, then tap Enable to turn it on or tap the recommendation to review the contents you can delete".

Hi,

in this article you can find informations that help make more space available in iCloud...

--- > https://support.apple.com/en-gb/HT204247#make_space

Settings app> General > iPhone storage is a good place to start