Struggling to completely grasp items vs sitemap definitions

If I understood correctly (which there is a good chance that I haven’t), then, when I declare an item linked to a channel of some device/thing, I am then also defining that channel’s type?


No. the Item has to be defined suitable to the channel(s) it is linked to.

For example, if you want to link a number channel, you have to use a number item (or a string item of course). If the item is in and out, the string item will also fail since even if sending a number as a string, this is no number, but a string with a number in it.
Of course there are some items which will fit, e.g. setpoint will also give a number.