A course has a fixed amount of content. Generally, no new content is added to it. You pay for the course just once. Other than that content, there are no other resources that are available with a course.
Membership is like a Netflix or amazon prime subscription. If you join a monthly plan, you pay the subscription amount every month and get access to ALL the content (courses, live classes, etc.). This process goes on till you stop the subscription. Hence the plan is called a monthly plan and not one month plan.
New content gets added to Art Insider Membership frequently. Every week's live class is an important type of content that gets added every week. Members can attend these classes live or can watch the recordings. The membership fee is a small amount that is paid at a frequency based on your plan. For a monthly plan, your card gets debited with the fee every month, for a quarterly plan, your card gets charged once in 3 weeks and similarly for a yearly plan.
The quarterly plan saves you 15% as compared to the monthly plan and the yearly plan saves you 25% as compared to the monthly plan. This is because you are purchasing the access in bulk and that is rewarded by giving a discount. You can cancel your membership anytime.