How do I disable that app bar at the bottom when switching apps?

That app bar you see at the bottom is Siri App Suggestions. This feature is believed to be safe—Apple encrypts and anonymizes all the data.

However, if you find it rather annoying, here is how you can disable it:

1. Open the Settings app.
2. Tap Siri & Search.
3. Scroll down to Siri Suggestions section.
4. Toggle the buttons to select where suggestions should appear (Suggestions in Search, Suggestions in Look Up, and Suggestions on Lock Screen).

Note: You can also disable Siri Suggestions for Individual Apps in the Siri & Search section.

Enjoy your iOS journey without suggestions anymore :)