npm commands explained

what is "npm install"
this command is used for installing javascript packagaes that defined in package.json,
this command must be run from a folder. that folder must contain package.json file

where is it defined
inside that file there is a key named "dependedcys" and its values are the packages that going to install in this computer,
installing means collecting package from internet or from locally saved files

to where is it installed
it is installed to subfolder named "node_modules" in the current working directory,
that means from which folder you run command prompt,

example
If your package.json contains:

{
  "dependencies": {
    "express": "^4.18.2"
  }
}

then when you enter

nmp install

express package is downloaded and moved to this path as shown below

node_modules/express