In our world, we organise in Pods - an autonomous group of 6-9 people with all the skills needed to solve a problem. Multiple Pods form a Team. Within a Pod, there can be multiple Senior Developers, but only a single Lead Developer. They have different and overlapping responsibilities and accountabilities.

Every project must have exactly one Lead Developer, and has one or more Senior Developers.

It is the accountability of the project or product manager to ensure that these roles exist in a team, and that the roles are filled with skilled team members able and willing to fulfil the role.