Several traits or indications I mostly come across with junior engineers is that guidance from seniors or on-boarding might be slightly more involved for them to bring them up to speed. Usually, they have less experience in working with production-grade environments or have some internships under their belt to make up for the entry-level experience gap. I would say that "years of experience" isn't enough to define/assess if an engineer is "junior" but my definition fits in with their ability of communication and critical thinking.

I would regard a "new grad" as "experienced" if they can objectively explain their choice of technology with evidence on their decision and if they can reason with the team with a proposal. Repeating this with good feedback and a ROI makes them experienced. Some projects in the open-source world are meritocratic, with a number of significant patches == commit access which makes them a trusted maintainer.