Components are specified using an element tag that matches the type name of the component. Think of Razor components like you would .NET types, because that’s  what they are.  For example, you can add a Counter component like this:

<Counter />

To use a component, the componet´s namespace must be referenced, example:

@using MyComponentLib

<Counter />

Note:  If you are going to use a component is several parts of your project, it´s a good idea to include de @using directives in the _Imports.razor file.

Also, you can speficy a component using its full type name:

<MyComponentLib.Counter />