Newcomers and new hires both simply mean new to a particular job. There could be a newcomer SRE with 15 years experience as an SRE.

Juniors usually implies generally no experience with anything. For example, I wouldn't call someone a junior if they had 15 years of Windows UI programming experience but for some reason decided to switch to a Linux / SRE careeer. But they would be entry level in their new field.