Hello,
Because the game is now bigger than 2GB it won't work as just an .apk anymore so you need the second file.
Download the second file (the .rpa file labelled as "2/2" on the download page) and then move the .rpa file (just the .rpa asset file, don't move the .apk file) to "/storage/emulated/0/Android/data/org.defending.lydia.collier.the66/files/game", then launch the game normally as you would via the apk,
How to move files can depend a lot on the specific device, here is a guide from Google and a generic video from YouTube, if it seems different on your phone you might need to search your phone's make and model and then "moving files", but this should work on nearly all devices:
https://support.google.com/files/answer/9808833?hl=en
https://www.youtube.com/watch?v=c4UwRaZ43hY
Hope this helps!
WP