I'm not sure which part of backend you're asking about specifically (since even the Tour goes into some detail on common backend processes, concurrency, and relatively common modules), but the Tour also references web application tutorials at the end, as a jumping-off point, which in turn can be tested on the Playground to an extent.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
I'm not sure which part of backend you're asking about specifically (since even the Tour goes into some detail on common backend processes, concurrency, and relatively common modules), but the Tour also references web application tutorials at the end, as a jumping-off point, which in turn can be tested on the Playground to an extent.