In Facer’s own documentation they note that “Due to restrictions of the Apple platform, Apple Watch faces currently cannot be fully customized and only ‘complication’ areas of predefined templates can be edited.”

They also mention certain refresh limitations: e.g., complications (third-party) can only refresh every ~15 minutes on watchOS.

So essentially, Facer uses Apple’s allowed “watch face template + complication” model, and injects designs into those templates rather than replacing the face system entirely.