imap Mail and iCloud vs local storage

Mail App

My sister has 25 GB of mail saved in folders in the "On My Mac" section of Mail App.

her 128 GB MacBook Air has only 2 GB of free space.

If she buy more storage on iCloud, and move those folder on the iCloud section of Mail,

Will She gain free space on the Mac,

I know that mail keep a copy of some Attachment but I wonder if everything is kept on the local disk, meaning she won't gain any space.

I am not sure.