You should pick a hypothetical target first. So you got the Tshirt store but I'd pick something more complex. And then go through all layers from the start and make your app modular and take advantage of 3rd party. For example for the Tshirt store. Use a cdn. All your pages should be ssr or issr if not ssg from the getgo. This reduces your server load. And off loads the brunt of your demand onto a 3rd party. (cloud flare, CloudFront)

Then use an auth provider and payment processor. (firebase, PayPal) with dbaas.

All that's left is glue and server less functions. And none of the scaling is something you have to do.

Once you get that then sure, peice by piece you can put it on your own hardware and think it through from there (cdn, db)