There are several types of collectibles in Little Nightmares. Similar to the hats in Little Nightmares II, the first part features collectible masks for the Six. They are obtained in different ways, and I'll describe how to obtain each in this guide.

This mask can only be obtained after destroying all 10 Geisha statuettes. Here is a guide on how to break all the statuettes:

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3011548129&searchtext=geisha

To unlock these masks you need to purchase Little Nightmares Complete Edition:

https://psteamproxy.yuanyoumao.com/app/584091/Little_Nightmares__Upsidedown_Teapot/
https://psteamproxy.yuanyoumao.com/app/584090/Little_Nightmares__Scarecrow_Sack/

These masks can be obtained by purchasing the following DLC:

https://psteamproxy.yuanyoumao.com/app/584092/Little_Nightmares__Tengu_Mask/
https://psteamproxy.yuanyoumao.com/app/584094/Little_Nightmares__Fox_Mask/

The Pac-Man mask is exclusive to the Nintendo Switch. Here are the instructions for obtaining this mask:

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2946860398
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2991094190
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2958599493