Smalltalk is three things in one. It is a language; it embodies a language design idea; and it is an operating system.
Learning Smalltalk gives you three things:
An understanding of Smalltalk, the language. This is least essential. It’s also the kind of thing you can pick up in a single 30-minute session.1 The language is tiny and simple.
An understanding of the design idea, uniformly object-oriented programming. This is crucial and will connect with other styles of programming including Actor-based and pure-functional. This is something you will never get from Java, which is not a uniformly object-oriented language.
An understanding of a completely different and (these days) unusual way of designing an operating system. An object-oriented operating system, to boot. The IDE, the debugger, and the other tooling all integrates to give a level of manageability in many ways far superior to e.g. Unix or Windows.
After a while, you may find yourself discovering subtle things about the interplay between the three aspects of Smalltalk that you can then apply in your own designs. For example, Smalltalk is a “dynamic” language, but the way classes and methods are arranged is very rigid, giving a lot of static structure to the system organisation. This static structure is what unlocks the powerful tooling, and is what simplifies the programmer’s mental model to make the system understandable. Comparable OO and almost-OO languages, such as Python, have a more “dynamic” system organisation, making it harder to write tools for automatically manipulating Python systems and making it harder for programmers to understand how Python code, data, state, and processes come together to form a running program image.