Hugo Kornelis explains how index scans work in SQL Server:
The logic of the Index Scan operator itself is fairly simple, but the actual actions carried out can vary hugely depending on the type of index being scanned (as defined in the Storage and IndexKind properties). Most of this logic is carried out at the level of the storage engine. Since an understanding of this is important to get a proper understanding of the performance of this operator, the actual actions carried out at the level of the storage engine will be described on this page as well.
The current version of SQL Server (2017) supports four types of index storage. The Storage property distinguishes between RowStore, ColumnStore, and MemoryOptimized; for the latter type only IndexKind further differentiates this into NonClustered and NonClusteredHash.
Scans are an important part of the database engine and knowing how they work helps us understand when they’re the right choice for the job.