Overthewire
Over the wire offers a lot of free and fun machines to learn to gradually access a Linux machine, gain a foothold and then pivot from there. It has several games catering to different stages of expertise.
This is basically a braindump of my notes on all kinds of topics: from going through several hundreds of hours of coursework over hands-on experience on setting up your own cloud infrastructure to everything that pikes my interest. This serves partly as a blog and mostly just to organize my brain and have my notes available and accessible.