Selecting multiple items from a connected board

Hi all,
I currently have a main board (let’s call it board A) which is connected to another Board (let’s call that board B). Board B has over 1000 items on it which can be selected from Board A. There are times when all the items on Board B have to be selected.
How can I do this without having to click on every single item?
As soon as you click on the item, it is selected so I can’t "select top item, hold shift key, select bottom item"
This is presenting as a major problem on a daily basis so I’d be really grateful if someone has a solution.
Thank you!