I did not vote to close, but if you see the reasoning behind a question being closed for "not being focussed enough", you'll find the following text:
Needs more focus - If your question has many valid answers (but no way
to determine which, if any, are correct), then it probably needs to be
more focused to be successful in our format.
This question currently includes multiple questions in one. It should focus on one problem only.
This can often be fixed by breaking the post into multiple questions,
or by focusing on a specific part of the problem.
Your question did indeed include multiple questions in one. Each of the 4 points you listed could have been addressed by user in an answer, therefore resulting in multiple answers partially addressing your whole post.
While you got an answer that satisfied your question, it is worth to point out that said answer may be considered far from exhaustive given the many example equations you had listed in your post. The answered just wrote the definitions of the symbols (except the $\cdot \nabla$ one that you also asked about, though it was addressed in a comment), but another answerer may have well given you explanations using the written out $\nabla$ in the context of hydrodynamics.
If all you wanted were the definitions of the symbols, you could have avoided the list of many equations (and slides) of hydrodynamics which may have led readers away from your main question, that was about the mathematical definitions of the symbols.
This is my interpretation as to why the question was closed for "needing more focus".