A CRM developer is a software engineer who builds, customizes, integrates, and maintains CRM solutions. CRM developers can build custom CRM software from scratch (this one will be fully tailored to your business and marketing needs) or customize on-premise software (which will ease management, lead to better data visibility and insight generation, etc.).
If you look for a CRM developer for building your own CRM system, you have 3 options. The first is to find freelance engineers, which will be a cheaper decision but put quality assurance and data security at risk. The second option is to hire in-house product engineers. This is a good solution if you have people with a tech background who can manage your project effectively. If you have no such employees, consider outsourcing. This type of cooperation will provide you with expertized product developers, effective management, and full control over the project.