yannseznec Is this a standard approach to output/input voltages in the eurorack world?
There's no such thing as a standard approach in Eurorack world, unfortunately. This means that any input should safely support any range between -10V and +10V, but may not behave usefully in part of that range. When there is a knob associate with an input, its behaviour is also not standardized, it may scale, add, or do something entirely different.
yannseznec more importantly: I'm confused about how to code with this in mind. in Pure Data if I multiply things coming into adc~ by two, then surely that will multiply both the knobs and the inputs by two?
Correct: Bela cannot distinguish between a CV in and a pot.
yannseznec and if I start using other modules, will they all generally use 0-5v? in which case, why are the inputs 0-10v at all?
In Eurorack you'll see voltages between -10V and +10V used in different ways. The analog outputs are straight from the Bela cape, with no additional amplification, in order to minimise part count, so they are limited to 0V:5V. Adding support for negative voltages at the inputs in combination with potentiometer scaling was going to make things complicated, so we only support positive voltages in the 0V:10V range to maximise compatibility with other modules.
If you are keen to have 0V:5V range for the pots as well, you can do so with a single component change: replace the ZENER voltage reference with a 5V one (e.g.: BZX79C5V10).