Monks and their manuscripts
The process of making books was a long and tedious process, only attempted and allowed to be made by monks. Usually done by candlelight in monasteries and abbeys, trained monks inscribed religious texts onto parchment later to bound into books or codices.
Monks and those of higher social class were the few groups who had the education to read or write making monks some of the most educated people of their time. Each word is written in ink on each page by hand before being carefully decorated with images, symbols and foliage then carefully gilded with precious metals such as gold or silver.
The script was usually done with a feathered quill or reed pen and was usually done in the popular blackletter of the time, which was bold and clear to see. The finished products sold for large sums of money as some took multiple monks, and months or years to make.
Sources:
https://www.ancient.eu/Illuminated_Manuscripts/
https://www.faena.com/aleph/articles/how-to-make-your-own-illuminated-manuscript/