iCloud and laptop storage

iCloud Photos is a syncing service. Photos are stored on the devices and on iCloud. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: https://support.apple.com/en-us/HT204264

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Videos and photos are treated the same.

re:

"(2) store things that you want to occasionally access but not have them use up your local device’s storage capacity. Am I wrong thinking that?"

Yes.

The only option is the Optimize Storage option discussed in Set up and use iCloud Photos - Apple Support

Also, as previously stated:

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

You could get a YouTube account and store them up there. The account can be limited to just those you want to have access to them. If you need to download one to your MBP there are apps that will do that for you like MacX YouTube Downloader.