So although I've been doing this for ~30 years as an untrained home barista on variously better and better kit ... I'm still learning stuff.

Single and double espresso really refer to the amount of coffee input into the basket ... these seem to vary but cluster around 9g for single ... 18g for double ... is 21g triple or 24g ... the biggest basket I was given was 21g.

Or does single and double mean more than just the input coffee, like something about the output too ?

If the basket states 17g on the side ... do I only put 17g in it ... or fill it to roughly the same volume each time ... because a really fine grind will occupy less space than a course grind ... so I'm tending to put 18g in my 17g basket ... should I only put 17g in always ? Maybe this has something to do with why the finer grinds stop the water flowing (>17g with less space) ... rather than still extracting (=17g with more space).

And the output ... 'ristretto' is 1:1 ... 1g coffee water = 1g/ml espresso ... 'espresso' is 1:2 ... 'lungo' is 1:4 ... is there a 1:3 ?