When Steamboat Willie enters the public domain, those aspects of the Mickey Mouse character that are in that film will also be in the public domain, so people will be able to use the Mickey Mouse character, as long as they don't use aspects of the character that were introduced in later (and so still copyright) works.

Something similar happened a few years ago with Sherlock Holmes [1]. The original Sherlock Holmes stories are public domain, and so the character can be used without the permission of the Conan Doyle estate. However, some of the later stories are still under copyright, so anything about the character which only appears in these later stories cannot be freely used.

[1] https://www.smithsonianmag.com/smart-news/sherlock-holmes-no...