What's the difference between a Junior / Intermediate / Senior developer?
Junior is capable but needs someone with more experience to teach, mentor, guide, or direct them. They are still learning, on the job, and you should expects the risks associated with such a person.
Intermediate can work on their own but isn't ready to lead, manage, mentor, or teach. These are the core of any team. They are proficient in their respective language and are probably learning others.
Senior devs are those beyond the bounds of a skillset. They may be leaders, teachers, or mentors in your organization. They can direct the priorities, roadmap, and business related to the development of your product. They are likely proficient in more than one technology, not necessarily other programming languages; perhaps they are familiar with database architecture, search algorithms and science, hardware and scalability, etc. such that they could run the entire technical organization of your company as a CTO.