Open class refers to a course that is open to the public on a public platform, that is, a course that can be attended by other teachers. The public is generally open to other teachers or outsiders.
Paid courses refer to courses that require money to attend, that is, courses that require money to listen. Generally speaking, paid courses are charged online or on site.
The content of the open class must be some simple and clear knowledge. Paid courses are deeper knowledge that will help you. If you pay for them, they will certainly give you the corresponding knowledge. If classes are held, most of them pay attention to the form of courses and the way of attending classes to attract people to sign up for study.