If you preorder from Origin, it will be added to your account automatically. It's going to locked until release time, then you can download/install. They will charge your bank closer to launch. In the past, they let those that have preordered EPs download a little early to avoid server traffic but we can't install & play until launch which will be midnight PDT converted to your own time zone I believe.
How to download: In the Origin client, My Game Library on the left side. Right-click The Sims 4 cover > Show Game Details (once the preorder is unlocked) Under Expainson Packs you should find The Sims 4 Pets to Download. Download times will depend on internet speed. Installation times will depend on drive write speed... but this usually isn't very long. It's not having great internet on top of possible bogged server slowing you down for download.