I think the answer to most of your questions can be found in the wiki.

https://horsefable.wiki/construction/ 

From the Wiki -

Feed Storage - Feed storage holds feed and automatically feeds horses at rollover. Horses boarded at home and with “Eat from Storage” selected from their page will be automatically fed if you have enough feed in storage at rollover.

Water Storage - Water storage holds water and automatically feeds horses at rollover. Horses boarded on your estate and with “Drink from Storage” selected on their edit page will automatically eat from storage at rollover.

So if your horses are boarded at home, you can buy food and water so they'll feed and water themselves automatically at rollover.

Oh man... Of course the one time I don't look at the wiki first it has the answer 😬 Thank you!