There's numerous techniques used. Many are covered in Gudeman's 1993 paper "Representing Type Information in Dynamically Typed Languages"[1], which includes low-bits tagging, high-bits tagging, and NaN-boxing.

The high bits let us tag more types, and can be used in conjunction with low bits tagging. Eg, we might use the low bits for GC marking.

[1]:https://web.archive.org/web/20170705085007/ftp://ftp.cs.indi...