Where do I start learning DevOps?
Devops relate to the execution of software. It is all about how to run an application or service. So server configuration tools, virtual machines and containers are the next steps. I suggest looking at the documentation of those tools and learning from them things like discovery, observability, scalability and redundancy. Just to name some examples:

Server config:


PXE
Ansible
Puppet


VM:


Vagrant
QEMU


Container:


Docker
Kubernetes


EDIT: Cloud technologies overview:


CNCF landscape


When your software architecture is about microservices, then containers are a good choice. When you are using old monolithic applications then VMs are not a bad option. So it always depends on the context what you need to learn specifically.
The topic is rather broad to summarize it. A book might help with that but sadly I cannot suggest a good broad one. "Kubernetes in Action" gives you a good start in Kubernetes though.