First of all, you need to be more specific about what you want to learn from the backend.
If you don't have guidelines, I recommend you check the backend roadmap, which will help you understand the backend.

Here I give you an overall suggestion from my personal experiences.
First, You need to select one programming language and learn it throughout. After that, move to backend frameworks. Here are some popular backend frameworks,
Programming Language | Framework |
Python | Django (recommended), Flask |
JavaScript | Express |
Ruby | Ruby on Rail |
PHP | Laravel |
Java | Spring |
Go Language | Gin |
All those languages and frameworks have good courses online. But here are some course suggestions that I tried.
Language & Framework | Course Recommendation |
Python | - Complete Python Mastery by code with mosh
- Complete Python Developer in 2022 by Zero to Mastery
|
Django | - The Ultimate Django Series by code with mosh
- Python Django 2021 - Complete Course by Dennis Ivy
- Python and Django Full Stack Web Developer Bootcamp by Jose Portilla
|
JavaScript | - The Ultimate Full-stack JavaScript Developer Bundle by code with mosh
- The Complete JavaScript Course 2022: From Zero to Expert! by Jonas Schmedtmann
|
Express Js (Node js) | - The Ultimate Full-stack JavaScript Developer Bundle by code with mosh
- Node.js, Express, MongoDB & More: The Complete Bootcamp 2022 by Jonas Schmedtmann
- Complete NodeJS Developer in 2022 (GraphQL, MongoDB, + more) by zero to mastery
|
I hope this will help you to learn backend development. Let me know if you need any help.