How do you collaborate with other developers to debug and fix issues in a shared codebase?

Use Version control to standarize your work within your team while maintaining a good track of status without the need to micro the team.

Using version control is the beginning of regaining control over the quality of the source code: With pull requests, you review and learn from each other's changes. With CI/CD hooks you automate testing and deployments. With branching strategies, you minimize breaking each other's code, and ease creating testing/QA versions of your system before you deploy to production. When you use version control effectively, code pushes to production are very anticlimactic because everything simply works. Without version control, changes to production systems are stressful gambles.

Use conventions, dont invent the wheel, use tools to help you stay in line with your team as "Conventional Commits" and follow recognized and standard git flows.

Coding standards are very important in general, but I would highlight that using the same coding standards enforced at the code editor level (eg, through editorconfig files) makes version control merge conflicts much less likely, since every coder is formatting the code consistently on their local machine. Many programmers have wasted much time and broken much code because they edited a couple of lines in a file, and then reformatted the whole file, which introduced merge conflicts, and then code was inadvertently broken as they resolved those conflicts. You can also check for code compliance as part of your CI pipeplines on your branches to make sure code conforms to the standard.

Communication is key. Its the base to construct a solid foundation to your leading strategy. Being opaque helps no purpose. Just embrace the team as a whole and retain from being granular with the problems. Take your time to think twice before deliver more entrophy to your team.

It's difficult to add testing to an already-built project. Start with the basics and build from there: 1) Add automated test execution to your CI pipelines; 2) Require successful test runs on all your branches before a pull request can be approved; 3) Add unit tests to new code written; 4) Adopt Test-Driven Design where your team writes tests before they implement their code so that all new code has unit tests; 5) When fixing a bug, first write the test to verify the bug exists, and then fix the bug so the test passes. Yes, it is difficult to add testing to a project that did not have it, but it's possible and realistic if you follow this incremental pattern.

Some people work well with pair programming, some do not. An alternative is to perform code reviews on each other for every pull request. If you have a senior engineer review a junior's code, they might spot weaknesses or unintended consequences. If the junior reviews the senior's code, they can learn about how to code for the system, and they can also ask questions about why the problem is solved this way. Code reviews when done in a non-critical way are an excellent opportunity to learn for everyone.

Constructive, non-critical communication is the key to efficient software engineer collaboration and team productivity. Having everyone adopt the mindset of asking questions and learning from each other's source (as opposed to criticizing and aiming to find errors and bugs) is extremely important. Having regular tech talks where people explain how they solved a problem (or how they are thinking about solving a problem) is also extremely fruitful, develops trust, and opens up people to communicating with each other more openly in general.