Welcome back to the Airtable community!
I doubt that many people have connected a vanilla html form to Airtable. Instead, people
Trying to connect your own html form to Airtable without a 3rd party service would involve using the REST API, and a huge amount of back-end work that isn’t very cost effective compared to the alternatives.
One of the issues with the REST API is you need to include your Airtable API key with every request. The CodePen example requires including your API key in client-side code. However, you should never include your Airtable API key in client-side code because then anyone could copy your API key and have access to all of your bases.
It is possible to hide the API key on the server side, but then you need to have server-side code, which is more complicated to setup. It’s do-able, but so much more work than 3rd party form services that it’s rarely worth the effort.