Instagram launched on a single server in 2010. 25,000 people signed up on day one. By the end of the first week, they had 100,000 users. Within two years, Facebook acquired them for $1 billion. The entire journey from one server to a billion dollar exit followed a predictable set of scaling stages that almost every successful system goes through.
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TL;DR: Every system scales through roughly 7 stages. You start with a single server, then separate the database, add a load balancer, introduce caching, move to microservices, shard the database, and finally add async processing. The key rule: never over engineer at stage 1 what you will only need at stage 5.
Stage 1: Single Server (0 to 1,000 Users)
Everything runs on one machine. Your web app, your database, your background jobs. One server, one process, one prayer.