Oracle SQL Developer Web is the web-based version of Oracle SQL Developer and it is included with Oracle REST Data Services(ORDS). You can check more on ORDS and what it offers here.
Oracle SQL Developer Web (also known as Database Actions) is a web-based application that uses Oracle REST Data Services to provide many of the database development and administration features of desktop-based Oracle SQL Developer.The main features include running SQL statements and scripts in the worksheet, exporting data, monitoring databases, and creating Data Modeler diagrams.
So if you have ORDS you will be able to access SQL Developer Web directly without downloading/configuring another product. What you need to do is to enable the ORDS feature during configuration on On-Premise installations:
Enter a number to select a feature to enable:
 SQL Developer Web (Enables all features)
More about ORDS configuration prompts are available in the official documentation.
The SQL Developer Web is not a new thing for ORDS, as it was available starting with ORDS 19.4, however from ORDS version 20.2 it is also known as Database Actions.
For Oracle Autonomous Database users the SQL Developer Web is already enabled and it is called Database Actions. Check the relevant sections for Autonomous:
New features in this release (20.3) include:
See The REST Pages
See The JSON Page
A presentation by the ORDS Product Manager Jeff Smith explains more on "what is" and "what you can do" with this feature: