Generate model in Rails using user_id:integer vs user:references

I’m confused on how to generate a model that belongs_to another model. My book uses this syntax to associate Micropost with User:

rails generate model Micropost user_id:integer

but https://guides.rubyonrails.org/active_record_migrations.html#creating-a-standalone-migration says to do it like this:

rails generate model Micropost user:references

The migrations generated by these 2 are different. Also, for the former, how does rails know that user_id is a foreign key referencing user? Thanks!