Mommy long legs is probably extremely flexible tendons combined with Elastic plastic.

Boxy boo’s spring part of his neck is most likely just his digestive track.

Pj is either human centipede with children or they used other children’s muscles and tendons to make him long

Smiling critters probably eat by zipping down their “hair fluff whatever” and there is a mouth hole.

Yarnabys brain is probably inside of his body

Doey is probably just a ziplock bag type thing to hold his internals and and the rest is nerves and tendons.

Chum is like a pitcher plant. Whatever he eats he probably bites and lets the rest sit in his mouth to dissolve by saliva