There are a lot of parts and roles involved with back-end development. It should be of no surprise that each response here is unique. You could make an entire

There are a lot of parts and roles involved with back-end development. It should be of no surprise that each response here is unique. You could make an entire career by developing deep specialization in just a part of what is discussed. Since you've said "back end" then that means you are considering client-server scenarios, potentially of the web application variety. If this were true, it's reasonable to feel confused and possibly overwhelmed by information. Because of this, many beginners have started with popular web application framework-platforms, such as Django or Ruby on Rails, and grown from there. This is a tried and true approach to learning back-end web dev. The concepts that must be known are those that apply to a role. For instance, a "web application developer" will likely be a generalist who is able to build an entire web app back end but may not have have ever opened the hood and rebuilt "the engine" or "the transmission" (car metaphors, not literal parts). A popular web framework will have a reasonably designed public API that abstracts away the complexity under the hood.