The full answer
Every change is made on a separate branch and submitted as a pull request. Another engineer reviews it for correctness, readability, security, performance and consistency with the architecture before it is merged. Reviews are also how knowledge spreads across the team, so no part of the system depends on one person.
Automated checks run on every pull request: unit and integration tests, linting, type checking with TypeScript where used, and dependency scanning for known vulnerabilities. A failing check blocks the merge until it is fixed.
Creuto's teams are senior-only, which raises the baseline. Experienced engineers make sound design decisions earlier and spot problems in review that less experienced teams would miss.
Shared conventions, such as folder structure, naming, error handling and logging, are agreed at the start of a project and documented. This keeps the codebase consistent as it grows and makes it easier for new engineers, including your own, to contribute.
Before major releases, vulnerability assessment and penetration testing adds an external check on security.