The template is using ISO week numbers, where a weeks starts on Mon and week 1 of the year is the week of the first Thursday of the year. So for year 2021, Jan 1-3 fall on week 53 of 2020. Hope I got that right, let me know if I messed up.
Update: I changed the sheet from published to shared. You can see formulas now, which hopefully is enough to create a 2022 template when the time comes. Comments are not visible, unfortunately.
Update 2: Changed sharing from “viewer” to “commenter”. You can see the comments now.