Take the time to go through all the options in the Settings app. There are lots of options, and doing so is cumbersome and time-consuming, but it's worthwhile for configuring the phone options to match your preferences. I've found the sections Notifications, General, Control Center, Accessibility, and Privacy & Security most influential for my preferences.
When typing, double tap the Space Bar to insert a period followed by a space.
Long pressing on app icons from the Home Screen will often give access to a few handy shortcuts. For example, with the Messages app one option allows you to go directly to the New Message screen.
The Firefox Focus app is a good set-it-and-forget-it ad blocker for iOS Safari.
Be mindful of how Apple benefits users who have multiple devices and take advantage as you see fit. Others have mentioned Continuity, the Notes app is good and syncs between iOS and macOS, the Reminders app will also sync across platforms, etc.
Don't count on Siri for much. Rudimentary tasks are typically reliable, but not much else: setting timers; basic math, saying, "Call Judy," tell Bob I'm leaving now (for a text/iMessage), basic language translations.
Selecting text provides a pop-up menu that can be scrolled right to left and has some helpful options (like getting a word defined and a few other things).
When the next major iOS release comes out in a couple months, Apple sites like MacRumors and 9to5Mac can be helpful resources for tips on the new OS.