They're stored just like any other value.
You can use DBCC PAGE to view data/index pages if you like. It's undocumented, but there are enough articles mentioning it.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass