I want some text appears whenever I create a new buffer with a certain file extension.

• vim-skeleton

  3 31 1.8 Vim Script

  Initialize new Vim buffers with file-type-specific templates

I use the vim-skeleton plugin for this.

• vim-snippets

  17 4,742 6.4 Vim Snippet

  vim-snipmate default snippets (Previously snipmate-snippets)

The plugin https://github.com/honza/vim-snippets has main templates for several languages, though without the #include in your example. They also place your cursor inside the function, rather than leaving a comment there.

• WorkOS

  workos.com sponsored

  The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.

  

• mu-template

  2 20 4.7 Vim Script

  Vim template-files expander

I'm maintaining mu-template. There exist many more plugins on the subject -- I remember a non exhaustive list on vim.wikia, now "fandom". Other people prefer pure snippets solutions where they manually request the expansion of the skeleton they wish to use.

Suggest a related project