"Editor" is a project-level role, not a workspace-level role. Workspace roles are Admin, Member, and Deployer. To assign someone as an Editor, go to the specific project's settings and add them under Project Members, where the available roles are Owner, Editor, and Viewer.

They have to create the project.

Workspace Members can create new projects, so your developer already has that permission. Once the project is created, you can assign them the Editor role from within that project's settings under Project Members.

so they stay as an Admin on my workspaces?

No, you don't need to keep them as Admin. You can change their workspace role to Member, which still lets them create projects and manage variables but removes access to billing, member management, and other administrative actions. The Editor role is then assigned separately at the project level from that project's settings. You can see the full permissions breakdown for workspace roles here.