Issue type schemes are templates or collections of issue types that can be applied to projects. As we can see in the following screenshot, Jira comes with a default issue type scheme, which is applied to all projects that do not have specific issue type schemes applied. When you create a new project, a new issue type scheme is created for you based on the project template you have selected. The new scheme will also have issue types pre-populated based on the template. As we can see in the following screenshot, we have two issue type schemes, TEST: Project Management Issue Type Scheme for test project and SCRUM: Scrum Issue Type Scheme not being used:

When you create your own issue types, to make them available, you need to add them to the issue type scheme that's used by your project.