Jina V3 is an embedding model, so it's a base model, further fine-tuned specifically for embedding-ish tasks (retrieval, similarity...). This is what we call "downstream" models/applications.

ModernBERT is a base model & architecture. It's not supposed to be out of the box, but fine-tuned for other use-cases, serving as their backbone. In theory (and, given early signal, most likely in practice too), it'll make for really good downstream embeddings once people build on top of it!