1. start with docker (or dokku), on a single server, it's easy enough to get going, automate CI/CD. Make sure your backups are working and relatively frequent.

2. Break your lower environments on to a separate server.

3. Break out your database, and set up redundancy at that layer. Leverage DBaaS if you can.

4. Grow to multiple app/api instances for redundancy, and setup rolling deployments.

5. Scale Vertically (bigger servers/instances)

6. Migrate to Kubernetes and expand your automation and tooling.

7. Break apart your application into smaller pieces to scale individually. Possibly leveraging platform tools like Lambda/Functions.

8. Work towards redundant datacenter and application data sharding to deliver a closer experience.

By the time you get to 5-6, you should be making money or have a good investor strategy in place for capital. There's very little need to go all out when at concept or earlier release stages.

6-8 may take place in a different order, depending on your needs.. but again, you should have money or have raised capital by this point, or you have a relatively good problem to have otherwise.

IIRC Stack Overflow grew vertically pretty big on a single server, then two (db/application split).