What happens when adobe plan expires

Yes, but the charge might be waived.

Please go to Adobe Customer Service (Make sure to sign in to your Adobe account first and allow popups and cookies on your browser. You may not be able to see the chat window if using a VPN.):
https://helpx.adobe.com/contact.html?rghtup=autoOpen will automatically open the chat window.
Type "Agent" in the chat box to bypass the chatbot and be connected to a person.

You may also be able to contact Adobe by phone:
https://helpx.adobe.com/contact/phone.html

or Twitter:
https://twitter.com/AdobeCare

if you have an annual plan paid monthly, yes -  you'll be billed each month and then next year your plan will automatically renew ad infinitim unless and until you cancel.

if you really have a monthly plan (which costs significantly more than an annual plan paid monthly), you'll be charged about the same, except there's no cancellation fee issue.

make sure you're aware of the cancellation terms by selecting your plan type (at the top of the page here), https://helpx.adobe.com/manage-account/using/creative-cloud-subscription-terms.html

for info on how to cancel your subscription, this page describes the few steps involved, https://helpx.adobe.com/manage-account/using/cancel-subscription.html

if you want to cancel but are unable to follow the steps described in the "how to cancel page", contact adobe support by using a browser that allows popups and cookies, contact adobe support by clicking here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human
inform the agent that you want to cancel.