I run https://extensionpay.com off a single server with a SQLite database. That feels kind of low-tech these days :) But SQLite has a lot going for it in production! https://blog.wesleyac.com/posts/consider-sqlite

The more high-tech part is that it's a SvelteKit app — so it has server-side rendering (for performance/SEO) with client-side hydration and other nice things. In fact, I like this stack so much that I'm going to spin it out into its own starter template so other people don't have to code all the tedious features that most apps have (like user auth, admin dashboards, etc): https://sveltesaas.com