I do both. Hard and soft copy, I write the concepts and features of the language in paper. While I do snippets of code, presentations (for myself), take down notes on Notes, (any note-taking app) for almost all of the rest that needs to be learn in the language. But what you can mostly do is to practice it in coding. The more you practice it, the more the concepts will be clear to you.

This ... isn't even something I ever considered. Back when I was learning programming we had these things called BOOKS, and in those books were all we ever really needed to know the ins and outs of a language.

But that was a different age, where you'd buy a Borland or Microsoft compiler and get a stack of four to six books of 200+ pages each with it. Even in the early days with ROM Basic you got a manual that gave a detailed reference of every command, an overview of the language syntax, and so forth WITH the computer or language so you never really needed a third party book on the topic. In fact Commodore and TRS-80's BASIC language guides were so simple even toddlers could grasp it.

... and who needs to write down notes when you have a clear concise reference that came with it?

Now though? Well, with PHP I don't really see the need since php.net has its own search, I use MDN as my JavaScript reference, and any other language typically has a website that gives me at LEAST a proper function/method reference.

You need a reference, we have the Internet.

For keeping strict code snippets, I use SnippetLabs, so I have syntax markup to help visualize code better. Any snippets I keep here are copied regularly to a static text file (for backup locally and on cloud), and project-specific READMEs (to help devs). I'm also getting into using VSCode snippets since I made the switch, but only with stuff I use immensely (like Bootstrap components or basic form elements) - since it requires learning and recalling shorthands (rcc anyone?).

When it comes to notation of process, or documentation of my own, I either keep track of what I'm doing step by step in Trello, write rough notes and snippets in MWeb, or write more loose notes in Evernote.

Trello's great because it supports Markdown, making things like code snippets and headers vs paragraphs more legible. And the fact my team can easily see the tasks I've completed, how I did each step, and replicate them using my snippets is cool. It's crazy simple to pop open a card, write out almost an entire article framework in Markdown, and copy paste that later into a Markdown-based blog or another longer-form text editor.

Evernote is great for jotting down ideas and syncing them across my devices. Although code snippets suck there, things like single quotes get converted to ` for some reason and break when pasted into code editors. I'm thinking of migrating to Markdown files and GDrive eventually, but it's hard to beat the convenience and accesibility of Evernote.

MWeb is great for writing more long form content (dark themes ftw), or translating documentation to a blog article. I also feel more confident placing my snippets/notes into an app that I know I can backup or export data out of, which MWeb makes easy (you can even generate a static blog out of notes - nifty).

When I was learning React:

1. I would open a new Evernote note for each subject/concept (Basics of React, React Router, Redux), and keep a document full of my notes, snippets, and links to articles/videos I was referencing.
2. After my rough notes I'd practice the concepts on my own, and during my workflow, I would copy snippets I'd use often to SnippetLabs or VSCode if I used them often.
3. And while coding, I'd make sure to keep extra notes in MWeb, like if I encounter any bug issues and reference specific StackOverflow responses.
4. After all that, I take the MWeb file and write a blog article describing it in full detail, so either my team or myself can understand the process.

I have no idea if this works for anyone else, but I have a bunch of files in dropbox where I take notes about whatever tech I'm learning. _git-notes.txt, js-notes.js, linux-notes.txt _etc. I also (separately) keep a library of little learning exercises if I've done them.

For programming languages I tend to construct a sort of personal X In Y Minutes type of document as I learn it. Basics like the syntax for vars, loops, interpolation; notes about the runtime; whatever is relevant.

Ultimately the thing is if you've spent some time googling it, writing it down helps solidify it; but also next time you hit the issue you might think "hey... did this before I think..." and you can go back and check. I like to write down the problem in my own words, then the solution including any specific terminology for the domain. Even if I have forgotten the precise terminology, I've found I tend to describe the problem in a pretty reliable/consistent way.

Plus it becomes a little code library. "Oh I need to do this with a string in this language... and there's my example from last time! yoink"

Maybe you can try making small code snippets that you think 'summarizes' what you have learned on that chapter and which you can reuse and build upon and save them in a folder in your harddrive?