Regular pro/rel is done first, with the 16 playoff winners in D.III replacing 8th place teams or losers of the relegation playoffs in D.II (4 from each league).
Then a bot cleanup is done. First any bots left in D.I are relegated to D.II. The ranking of teams during the cleanup is:
1) Regular season finish (1 to 8)
2) Regular season wins.
3) Point differential.
4) Points scored.
The process is repeated for each pair of divisions, but in the case of D.II and D.III any bots that relegated from D.I by regular relegation or part of the bot cleanup are then relegated to D.III, and so on.
In your case, the team that promoted was a 1st place team with a 22-0 record. In coming seasons it will probably require a 1st place record and a very good record to promote as part of the bot cleanup from D.II| (in the USA it required an 18-4 record and a very good point differential).
From D.IV was much easier, with 2nd place teams with 17-5 records promoting in the USA. In past seasons 3rd place teams had promoted from D.IV as part of the bot cleanup.