Microsoft Graph connectors allow you to ingest 3rd party data into Microsoft Graph which can then be surfaced in native M365 experiences as well as programmatically through Microsoft Search APIs. Please see below for Microsoft Graph connectors developer docs on creating your own custom connector. Note that the tenant will need to have sufficient item index quota via licensing.
Graph connectors dev docs
https://aka.ms/graphconnectorsapi
Graph connectors overview / admin docs
https://aka.ms/graphconnectors
Graph connectors licensing information
https://learn.microsoft.com/en-us/MicrosoftSearch/licensing