What are some good resources for learning back end web development?

Not sure exactly what you want to learn on the back end but if it's just learning than I think one of the best ways to go is get a Raspberry Pi and start tinkering with it.

You can also spin up a VPS on DigitalOcean and do much the same. They have some very good tutorials on installing and configuring web servers and SSL certs and DNS records, etc. All kinds of things really.

Personally, I use Perl to manage server side chores for things like processing PayPal payments and setting up and managing user accounts, but I'm pretty sure python would work just as good.

With Perl I use modules like LWP, JSON, Net::SMTP, and even CGI.pm to deal with simple html form data.

I also use CouchDB as opposed to an SQL DB. That's a personal preference but you can do a lot with that on the server side with Perl and CURL, the terminal, etc.

I've found https://www.theodinproject.com/ really valuable. The resources they've gathered have a good balance of fundamental theory & practical exercises.