Most illuminations I see depicting medieval scribes are them writing on already bounded books (Image 1, Image 2, Image 3). But this isn't intuitive for me because I don't think they could calculate how many pages they were going to write so in my opinion it is more logical to write first and see how many pages you use and then bind together everything you wrote (something like here). But at the same time in the last verses of Dante's Comedy (can't remember if Inferno or Purgatorio) Dante says something in the lines of "I don't have any space left so I need to finish this section now" which may support that scribes wrote after binding them.
If they wrote books before binding them how could they possibly calculate what they were going to write beforehand?
Terminology for manuscripts used here:Folium (pl. -a) a single leaf, front and back (two pages); Bifolium (pl. -a) a single sheet which is stacked in a quire (pl. -s) of usually 4-5 bifolia, and is the basic unit of the sewing structure of a manuscript
Keep in mind that medieval images are often not strictly representational, but often show things either as symbols (a finished book for the book that is being written) or even at different stages in time (fencing manuals will sometimes do this). Medieval scribes and illuminators generally worked in unbound quires, which are much easier to handle and do not have the problem of the bound gutter (the part where the fold is) being hard to write on (see this image from Royal MS 19 C II, f. 48v). Some would be written on a type of sloped writing desk, as pictured in your post, and some would have written on their knees, as in this image of an Ethiopian scribe at work (the Ethiopian tradition is not the European tradition, but they are related, and this is shared). The quire itself was then 'tacketed' together with a little piece of parchment used as thread or the like so that the sheets would stay together for binding. Numbers or the last word of the proceeding quire would often be written on the front to allow them to be put in the right order for binding (making it very clear that they were written before binding).
Edit: it is worth noting here that many written works would probably have existed as quires and never been bound, and these are largely lost, except when they were later bound together with other works (a 'Sammelband'), where we can see from the wear/dirt that they circulated separately and in many cases unbound. Little notebooks and material for personal use, like notes on homilies for preachers might often have never been bound.
Quires formed a set unit of writing that was even farmed out at some point to different scribes, in what was called the 'pecia' (piece/piecework) system, where the scribes would be loaned or rent a specific quire and then produce that same quire, so we know that they were able to do basic calculations on how much space they had/match the amount of space in the exemplar (the thing they copy from). If needed, they could adjus tthe amount of written space by adding or excising a single leaf from the quire. I don't know much about Dante, but he was probably not composing in what would be a finished manuscript, so it is hard to say what is meant by that in terms of drafts (perhaps a Dante scholar would know), but he may have been approaching the end of the writable material he had on hand (in the earlier middle ages, wax tablets were used for drafts, but I do not know Dante's practice), or perhaps he had planned how much space to use.
Some medieval books do get written in directly, especially the late medieval/early modern genre of miscellanies or commonplace books, which are kind of catch-all whatever-interests-the-owner books; often quotations/short excerpts of writing, but also recipes (a famous one is a party trick for making a giant egg), and even magic spells.
Suggested Reding:
Clemens and Graham, An Introduction to Manuscript Studies
Alexander, Medieval Illuminators and their Methods of Work
Literally anything by Christopher DeHamel. A short intro is the British Library crafts volume, Scribes and Illuminators
Houghton, Recipes in Manuscript Miscellanies, The Recipes Project https://recipes.hypotheses.org/8572
