A print interior carries its fonts inside the PDF, and a font's licence says whether that's allowed in a book you sell. The studio sets books only in fonts whose licences allow it: EB Garamond for the text, Atkinson Hyperlegible Next for the dyslexia-friendly style, and Shippori Mincho and Zen Kaku Gothic New in Japanese editions, each under the SIL Open Font License 1.1. Should a character be missing from them, the typesetter's own fonts fill in, under the SIL Open Font License, the GUST Font License or the Bitstream Vera licence. A cover's words are drawn into its picture in the same type, and the studio's barcodes carry their digits in DejaVu Sans Mono, under the Bitstream Vera licence.
Every print build checks the fonts in its interior against the licences on file, and its report says which licence they're under. Should a font the studio has no licence on file for ever turn up, the build says so: write to us before you upload that file.
Ebooks carry no fonts: e-readers show the text in the reader's own.