PCSX2 uses two different ways to store data on these "virtual" cards:
If you want to change where these files are stored (for example, to a cloud-synced folder like Dropbox or Google Drive), you can go to Settings > Folders pcsx2 save files location work
You can "eject" a full card and insert a new blank one to get more space, just like on a physical PS2. PCSX2 uses two different ways to store data