Kendra Little has a guide for us:
We’ve recently updated the SQL Server and Azure SQL index architecture and design guide. This article is an in-depth guide to indexing in databases using the SQL Server engine, including SQL Server, Azure SQL Database, Azure SQL Managed Instance, and Azure Synapse Analytics.
Our recent update adds a table to categorize the types of indexes discussed in the article, clarifies B-trees vs B+ trees, and describes how row locators (aka “secret columns”) are used in nonclustered indexes.
Read on for more information and do check out the guide.