Run Apex tests locally
Install Nimbus, point it at your SFDX project, run your existing @isTest classes. Five minutes from "I have never tested Apex outside an org" to a green test suite on your laptop.
Apex CI without a connected org
GitHub Actions and GitLab CI walkthroughs. No JWT cert, no DevHub, no scratch org pool. JUnit XML and Cobertura plug into Codecov, SonarQube, and the rest of your CI tooling.