Sanity is one of a number of emerging content-as-a-service platforms, also known as “headless CMS”. Another one that I have some experience with is Contentful. These services are essentially content + API together, able to feed into multiple client front ends. It used to take 10s or 100s of hours to get something like this up and running and scaling, so for content heavy projects, this is much simpler and much less time spent building the basics.
Sanity have their own GROQ query language. I found their writing on What About GraphQL? to be useful.
You develop schemas for your content and then they deploy a full editor / CMS or “content studio” as they call it, which gives you a local or fully hosted editing environment based on the schema of your content. The content studio is open source: https://github.com/sanity-io/sanity
Pricing at the developer/personal project is free to start with basic usage – 100K http requests, 3 users, 5GB assets, 10K documents (a blog post would be one document, but so would a Category/Tag or an Author). There are reasonably upgrades in the tens of dollars per month for more. The first “pro” tier is $200 per month.
Not really in the same class of things, but the Netlify CMS also gives you a web-based editing environment that commits to Git, also based on a schema of your content.
Aside: that is an awesome embed from Sanity (entering in the link to the home page just adds that video automatically).