When to Describe One, and When to Use a Template
A custom interaction is not an upgrade you apply to a lesson. It is a different kind of work, and the honest default is that most screens should still be templates. The 70+ template library exists because the shapes in it are the shapes most training needs, and a template is a known quantity: it is already responsive, already brand customisable and already behaves the way the last course that used it behaved.
Describe a custom interaction when the activity is the content. Technical training where the learner has to repair something rather than recognise it, a calculation your people get wrong in the field, a sorting task that only makes sense against your own categories, a judgement call whose consequences you want to play out in front of them. In those cases the template version is a compromise and everybody involved knows it. This is also where a well-described activity earns its keep for years, because the thing being tested is a real task rather than a phrasing.
Reach for a template when the shape already exists. If the activity is "pick the right one of four", "put these in order", "find the thing in the picture" or "recall the term", the library has it, it has been through every browser you care about, and describing a bespoke version buys you nothing but another thing to check. The same goes for the lesson you are building at speed: our guide to rapid authoring makes the same argument about the wider build, which is that production time is worth spending in a few places rather than spreading evenly.
One rule holds for both. Whatever the lesson is made of, read the per-question results afterwards. Learner management reports completions, pass rates and individual question responses, and the question people keep getting wrong tells you whether the activity is hard or just badly described.