Do I need a degree or is a diploma enough?
A diploma in the relevant engineering branch is sufficient for most posts, which is what makes this route valuable to polytechnic holders. The accepted branch varies by post and department, so check the current notice.
How deep is the technical content?
Diploma level rather than degree level, which suits the qualification the post requires. Revising your polytechnic fundamentals thoroughly is more productive than studying advanced material you will not be asked about.
Is the state Junior Engineer exam the same?
No. Several states run their own recruitment with different syllabi, patterns and calendars. If you are preparing for both, check each syllabus separately - candidates lose marks assuming they overlap more than they do.
What is the actual work?
Supervising construction and maintenance, estimating and measuring work, and checking contractor output against specification. It is a field and site role with real responsibility for public assets, not a desk posting.