There are a few ways to make a PDF.
The oldest, and one that still works well, it to Print from the app. When the Print dialog shows the document preview, pinch out to expand the document to full(ish) screen. Go to the Share dialog (again) and this time choose Save to Files.
In many apps the Share dialog has a Save as PDF option. It works just like Print, but with fewer steps. I don’t know what the criteria is for this option to show up. If it is hiding or obnoxiously low on the list, go to the bottom of the list and choose Edit Actions. You can add it to the top items to make sure it always shows near the top (when available)
In many other apps you will find the PDF option in the Export options instead of the Save or Share options. This is where it is in Pages.
You can do some custom PDF creation using Shortcuts app and the Make PDF action. By itself it’s not very useful, but it you had a lot of PDF’s to create it could automate the task.
If you happen to have/use Readdle’s Documents or PDF Expert apps, the paid versions have PDF conversion and merge tools. I’ve been using this recently to combine scanned images (of documents) and the .txt files I’ve been creating for our club’s old records.
I’m probably missing a few methods.