Why do you need to do yarn install after npm install?

Question

npm

The situation is this, in one project (api server on express) I put the npm install some-package module (I tried 5 different pieces), then I run yarn dev and nothing - cannot find module 'express'. But if you do yarn install and then yarn dev - everything is ok. Somewhere in the corners of the subconscious, I understand the reason, but there is no clear picture. Explain to the newbie why this is happening?