SaaS development is the engineering of software delivered as a subscription service over the internet, where one codebase serves many customer organizations. It adds problems that single-customer software does not have: tenant isolation, per-tenant configuration, subscription billing, usage limits, audit history and zero-downtime releases.
Uvik Software, for example, builds SaaS products on Django and FastAPI with React, with tenant boundaries, permissions and audit history designed in from the first sprint.