These are some useful resources I have learned from that I would recommend to others.

Software Architecture

Martin Fowler Website
12 Factor App (Heroku) Article

Domain Driven Design

DDD: The Good Parts Video
Domain analysis (Microsoft) Article

Microservices

Principles Of Microservices (Sam Newman) Video

Agile / Workflow

Agile manifesto Article
Agile principles Article
Continuous Delivery (Dave Farley) Video
Code Review Love Article

Git

Pro Git E-Book
Fix up and autosquash Article

C++

C++ Core Guidelines Webpage It is not important to memorize the recommendations because most of them can be enforced using a static analysis tool, however, it is good to understand the rational behind the guidelines.
C++ FAQ Article Helps to explain the language design.
CppCon Youtube Channel  
CppCast Podcast Hit or miss depending on the guest. Good for keeping up to date with what is happening in the C++ ecosystem and hearing real world stories.

CMake

Effective CMake (Daniel Pfeifer) Video
Professional CMake (Craig Scott) E-Book

Python

Transforming Code into Beautiful, Idiomatic Python Video A few tricks to write more idiomatic Python which usually results in less lines of code and faster execution.

Documentation

Technical Writing (Google) Online Course A short and well structured course.

Data Structures & Algorithms

Abdul Bari Youtube Channel  
Introduction to Algorithms (MIT) Youtube Playlist  
Leetcode Website Lots of brain-teaser problems that typically require knowledge of data structures to develop an algorithmic solution.

Operating Systems

Unix System Calls Part 1 & Part 2 (Brian Will) Video
Containers From Scratch (Liz Rice) Video
Microkernels (David Evans) Video

Networking

Networking tutorial (Ben Eater) Youtube Playlist Great explanation of networking from the physical layer up to the transport layer.
How TCP Works Youtube Playlist  

Programming Paradigms

OOP

Object-Oriented Programming is Bad Part 1 & Part 2 (Brian Will) Video
SOLID Principles Article
GRASP Princples Article
Dependency Injection Demystified Article