What are some good sites or blogs about the mechanics behind trading?

I used to work as a professional trader. I see this question so often, but it's just a strange question to ask. You are asking something like "I want to learn programming, you know, to make stuff like Apple or Google" - If you ask a dozen developers they will give you a dozen different answers depending on what their background is. Some will tell you to learn Javascript and React, some will tell you to understand infrastructure and Go, some will tell you to look at REST and Ruby on Rails, and some will tell you to look at Flutter for making mobile apps, and so on. None of them is wrong, but you're getting all these wildly different answers because you're asking an imprecise question.

Trading is the same. There are dozens of different types of "trading" and everyone you talk to will have a slightly different take, depending on their background. They may talk about portfolio management, directional trading, market making, HFT, arbitrage, options trading, exotics, long-term investing, all of this over different time horizons, technical analysis, fundamental analysis, and so on. None of them are wrong and all of these things are related (just like REST and JS may be related), but you need to be more precise in what you are asking. Someone trying to manage a diverse portfolio and trying to get a low-risk return over several years is using a completely different set of technologies and methods than someone working in HFT. It's like comparing someone writing low-level C++ driver code to someone iterating on HTML and CSS. It's just different, but both are programming.

As a side note: 99% of what you find online are gurus trying to sell you something. They are making a living selling and re-purposing educational content, but have never built anything profitable themselves. And even if they have, what worked 10-20 years ago is no longer relevant today. Books that teach fundamentals are generally of higher quality. [0] and [1] are a good starting point, but again, they are quite specific to certain types of trading.

[0] https://www.amazon.com/Trading-Exchanges-Microstructure-Prac...

[1] https://www.amazon.com/Concepts-Practice-Mathematical-Financ...

All the money in quant investing is currently coming from analyzing data sets that are not publicly available. Things like credit card data, mobile phone locations, receipt data, etc. When those are crossed with traditional economic indicators and equities prices, a fuller picture emerges.

When a data set is updated, i.e. credit card transactions come out for last week, retail stocks will move accordingly. Modeling this as an outsider without access to that data is like trying to predict a black box.

Have you tried https://blog.quantinsti.com/

Your question got me thinking too and I found that on page 3 of DDG.

It's something I'd quite like to know abit more about too.

Google Ernie Chan trading. I don't agree with the trading (I am pro investing) but Ernie explains as a practitioner