If you're hesitant about storing all your photos and other apps data on iCloud you can selectively give access only to Maiar.
Just follow these simple steps:
Click the "Settings" app on your iPhone
Click your name at the top of the screen
Click iCloud and make sure you are logged in
On this page:
Scroll down and first enable iCloud Drive.
Then select (or deselect) all the applications and services you want to be stored on iCloud Drive.
If you only want Maiar, scroll to the bottom and enable.
Learn how to set up iCloud on all of your devices though this guide.