In the linked video at 1 minute 20 seconds, if one stops the video, a typecase can be studied.
In printing with metal type, each individual piece of metal type is known as a sort.
Please note that in the typecase, the compartments are not all the same size.
This is because in each collection of sorts of metal type, known as a fount (note the spelling, differing from the font of computer fonts) there are more sorts of the letter 'e' than of the letter 't' than of the letter 'a' and so on.
With computer fonts, one gets one of each sort, because as many as one needs can be copied electronically, but with metal type the number in each fount varies depending upon which letter it is.
The letters used more often in text not only have a larger compartment in the typecase, but the compartment tends to be nearer to the centre of the typecase.
LINK > Gutenberg Museum in Mainz (2 minutes 29 seconds)