What is the difference betweene 'npm i' and 'npm install'?

There is no difference, since "npm i" is an alias for "npm install". They both do the exact same thing (install or update all the dependencies in your package-lock.json).

Docs: https://docs.npmjs.com/cli/install via CRice

npm install (aliases: npm i, npm add ) (with no args, in package dir)
         npm install [<@scope>/]<name>
         npm install [<@scope>/]<name>@<tag>
         npm install [<@scope>/]<name>@<version>
         npm install [<@scope>/]<name>@<version range>
         npm install <git-host>:<git-user>/<repo-name>
         npm install <git repo url>
         npm install <tarball file>
         npm install <tarball url>
         npm install <folder>