A template is one piece you will want in more than one lesson - a header, a footer, a layout. You build it the way you build a lesson, and then add it to whichever lessons should carry it. It is not what decides a lesson's overall shape: a lesson imported from a book gets that from its own content, with a Lesson and Discussion tab where there are discussion questions and a single flat page where there are not.
Open Library, then Templates, and press New Template.
Give it a Template name, choose its Type - Header, Footer or Layout - and press Create.
It opens for editing, built the way a lesson is. Press Save Template when it is done.
To put a header or footer on a lesson, open that lesson and choose Settings, then Templates.
It is added at the top or the bottom, and you keep or undo the change after seeing it. A Layout cannot be added to a lesson this way.
Adding a template to a lesson COPIES it in. Changing the template afterwards does not change the lessons that already carry it, and deleting the template does not take anything out of them.
There is no way yet to put a template on every lesson in a book at once - it is one lesson at a time, from that lesson's own Templates menu.