There are at least three ways you can reuse training texts/pages.

First way is what you did now: simply copy paste content from one page to another. But when you do that, please copy from the editing page, not from the viewing page. This method has the huge disadvantage that you would have to remember where all these copies are and modify them individually when you want to change something. I would not recommend continuing like this

Second way and probably the simplest and most suitable way for what you want to achieve now is to create "course content" page for each individual exercise or group of exercises that you want to reuse. You did this now already - you have the exercises. If you want to sell them in a bundle, the next step is to create a course and associate all the materials to that course. It's pretty simple. The only disadvanatge is that the user will need to access each and every exercise page separately from their profile page.

Third way, the most flexible and future proof, is to create a text snippet for the text that you want reused and use it in multiple courses. The advantage of this is that you can freely create the structure of each course and you only have to modify the text once if you make changes.