If I buy additional iCloud space for my iPhone will it be available for my iMac and MacBook Air? Do I need to buy extra for each device?

iCloud storage is automatically available to all devices using the same Apple ID for iCloud. It can also be shared -- see: Share iCloud+ with your family - Apple Support

An iCloud account is tied to an AppleID, not any particular device. Any device signed into iCloud with the same AppleID accesses and uses the same iCloud account and storage.