Apple Mail Local Storage vs Cloud Based Folders

Hello all,

I have a long question regarding using local storage in Apple Mail vs "cloud" or "server" storage. My mom has several email accounts that are active in Apple Mail. She has several gmail and one icloud account. She has all of her subfolders like a folder called personal, family, receipts, online orders, etc. listed under the section called "on my mac" in Apple Mail. Inside each of those folders, she has several emails from multiple email accounts. How can I, in a simple way, explain to her A.) what icloud is and what it does, and B.). how she can get her emails out of those locally stored folders and move them to cloud folders for each account? Am I correct that she would have to move all of her emails out of those folders back to the inbox, create those folders for each email account, and then move the emails back in to those folders under the server account in the appropriate folders per email account? In other words, she has, for example, a folder called online orders, however, she has an email from an online order that was sent to "X" email address and an email from an online order that was sent to "Y" email address. Am I correct that she'd have to make sure she puts the "X" email from her online orders folder to the online orders folder under "X" email address and then the email from an online order that was sent to "Y" email address and drag that email to the online orders folder under "Y" email account? In other words, she'd have to do that for every single email that's stored locally in those folders under the "On My Mac" section? So that she'd be using cloud based folders and be able to download her entire email account, sub folders and all, on any device she has? By the way, she is not able to search for any email via portal, whether it be for gmail or icloud, because all of her emails that aren't in her inbox are stored locally on her laptop under "On My Mac". She'd have to move any email to her inbox to be able to search for it via web portal.