Unresolved function or method mount(), createApp() and type Vue

I am new to Vue and using Webstorm. Do I need to properly create a new vue app using “vue create my-vue-app” or I can just using Vue methods in normal html and js files? I did install npm and use it to install Vue.

I noticed that when using Vue methods in normal js files, Webstorm give me warnings like unresolved functions or methods like mount(), createApp(), and the type Vue. It worked fine when I opened the html file in browser though. I just don't like the yellow underline and the method/type name not colored.