Fairly easy to do (I tested it):
Go to the folder you want to copy the ships FROM. Anything in brackets, below, is a variable you'll need to recognize.
C:\Users\[YourWindowsLoginName]\AppData\LocalLow\Intercept Games\Kerbal Space Program 2\Saves\SinglePlayer\[YourCampaign#1]\Workspaces
Each ship/build will have three file types: JPG, JSON, and META. Select and copy all three files for each ship/build you want to transfer.
They will have names you gave them when you saved them.
If you didn't name them, then they'll be marked with "AutoSave [Campaign Name][Date/Time]"
Go to the folder you want to copy the ships TO.
C:\Users\[YourWindowsLoginName]\AppData\LocalLow\Intercept Games\Kerbal Space Program 2\Saves\SinglePlayer\[YourCampaign#2]\Workspaces
Paste your copied files into this [YourCampaign#2] Workspaces folder.
Run the game.
That's it.
Sitting Duck's post in Transfer craft between two campaigns? was marked as the answer