When developing an application the beginning is always easy. There are no existing features which can be broken by the addition of a new feature. After the application reached a certain level of complexity it gets more difficult. The developer needs to know all implications of the new changes throughout the whole code base.
Luckily there are a couple of ways to mitigate these issues:
Separation of concern: The application gets structured into layers, which have their own focus. For example a database layer which encapsulates all DB access. If the DB needs to be changed, only this layer is affected.
Linting: Using a linter to get rid of all syntax warnings. A lot of unimportant warnings can drown-out important ones.
Code quality/best practices: Many languages have tooling to detect code smells or the use of old language features, which have been replaced with modern ones, which very often are more performant and safe/secure.
Dependency/tooling management: Keep precise track of every dependency version as well as the version of every build tool. Makes builds more "reproducible" and avoids subtle issues if the code is checked-out on a different machine and compiled with slightly different dependency versions.
DB migrations: Using tooling to manage the DB migrations. Less important during initial development, very important after the first release.
End-to-end test suite: A comprehensive test suite covering the whole application. Used to identify regressions. Plays the role of a "test user" of the application.