I would probably have to say YouTube is the worst place to go for asking a question (based on a video). If you ask a question in the comment section, you either get ignored for an eternity or get stupid answers such as "Darude - Sandstorm" ... & that's no joke. If you're very lucky, you might get a halfway decent answer, but don't hold your breath. Speaking from experience, you hardly ever get a good reply & you almost never get an answer from whoever posted the video. And especially don't expect YouTubers like Jacksepticeye to reply, because they're way too popular to answer each individual question they get on their videos... -_-
Microsoft and British Telecom. (the latter being my ISP)... unless you want to buy something!
The on-line "manual" from the 'Help' function on the TurboCAD draughting programme I am trying to learn.
Most mathematics text-books if you want to understand any particular maths topic.
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Though to be fair I discovered something completely unexpected about the TurboCAD manual, a pdf document. Its layout is rather haphazard and has no Search function beyond page-number.
What was unexpected, given that pdf format, was finding it possible to copy the Contents page into Word, and after a lot of editing in that, produce a printed version in full alphabetical order, using Excel. This means I can now find specific topics in a fraction of the time it took previously. Doesn't of course mean the topic itself is any better explained.....