The place to start on your storage cleansing journey is Settings > General > iPhone Storage. Here, not only will you get an overview of what's eating up your storage (apps, system, photos, media, and other), but you can also do a deep dive into the apps that are using the most data.
Here I'm learning that I have too many photos (or memes) saved, too many audiobooks downloaded, been flying my drone a lot, and Twitter, YouTube, and WhatsApp, and Facebook are each taking almost a gigabyte of storage.
Tap on an app in the list and you get a deeper dive. Here I can delete the app and free up all the storage or offload the app and keep the documents and data on my iPhone.
Sometimes I get the ability to see the data that's stored by the app and get the option to manage it (the DJI Fly app gives me this option).
Other apps don't give me any way to delete the data beyond deleting the app. If I don't want to delete the app then I'll have to fire it up and see if there's a way delete data or clear the cache.
In my example there was a Manage Storage feature in WhatsApp that let me delete videos, and I had some downloaded videos in YouTube. As for Twitter, I deleted and reinstalled the app and freed up a lot of space.
A recommendation that iOS is giving me is to offload unused apps and free up almost 8GB of space without losing data. I take iOS up on this. It's just one click, so a no-brainer. If I have to download something massive again down the line, but I'll live with that.
Apps that have been offloaded get a little cloud icon with a down arrow next to their name to indicate they need redownloading.
This brought me back to having well over 10GB of space free, so that was good enough for me.