Because objects in Smalltalk encapsulate their representation, it's not possible (without using reflection) to figure out exactly how an object represents itself. So things that would be primitives in other languages, like small integers, are in Smalltalk ordinary objects like anything else. The language works hard to present a 'uniform object model'.

However! When the rubber meets the road, there has to be some kind of bit-level representation somewhere. In Smalltalk, the VM hides these details from the programmer, who gets to reason uniformly about objects without caring about their representation. Specifically, Smalltalk VMs often use pointer tagging [0] just like lots of other dynamic languages do.

[0] Lots of interesting stuff on pointer tagging here: https://www.snellman.net/blog/archive/2017-09-04-lisp-number...

If you are familar with Java: Just imagine all integers are objects from class Integer and that int doesn't exist.

(Internally, they are actually represented as 31-bit numbers for performance reasons)

Via compiler magic, conceptually you use all types the same way, but the compiler has specially cases for a couple of them, thus handling them as primitive types, even though you cannot tell the difference.

For example in Eiffel is even more complex, when you create a class it is possible to tag it with the default behaviour (reference or value), which you can later override when declaring variables or type parameters in generic code.