15
How does indexing work at huge scales?
(programming.dev)
Icon base by Delapouite under CC BY 3.0 with modifications to add a gradient
Often it's distributed databases like Google spanner, amazon aurora, cockroach db, etc. Google has a public whitepaper on spanner you can read.
https://static.googleusercontent.com/media/research.google.com/en//archive/spanner-osdi2012.pdf