If you haven't already, play through the Spring levels using this guide:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3017500512
Once that's done, turn on Creative Mode in the settings on the Main Menu.
You'll need it for these solutions to work.

Complete all the levels.

View Winter 20 to unlock The end?
View the post Winter 20 credits scenes to unlock They live among us!
Return to Winter levels screen to unlock Winter

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For my other guides, they are here:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2880052179