Question: How to export Oracle DB data using SQL Developer? I need all data, tables, constraints, structure and so on.

Answer: 
If, at some point, you only need to export a single result set, just right click "on the data" (any column of any row) there you will find an export option.
The wizard exports the complete result set regardless of what you selected
To export data you need to right click on your results and select export data, after which you will be asked for a specific file format such as `insert`, `loader`, or `text` etc. After selecting this browse your directory and select the export destination.