Short answer: Yes, Shopify backs up your entire shop. Longer answer: Why do you need a backup?
Your shop's products, collections, orders, customers, themes etc. are all backed up by Shopify so that in case their storage fails they can switch to new storage without losing data.
This backup is invisible and seamless for you as a store owner. The backup is entirely for Shopify to be able to keep your shop running, you have no access or control over it.
It's good to know that Shopify has you covered in case of these kind of storage failures. But is that all you need a backup for?
What if someone makes a change to your theme that breaks it? Or deletes something by accident? In cases like these Shopify doesn't give store owners the ability to recover an older version of your entire theme, products, collections etc.
In case of problems and mistakes like these you should have a good, clean, recent backup on hand. At the most basic level, before making key changes to your shop use the built in export features to save this somewhere safe.