Here are some articles you can read on Microsoft Docs about control add-ins:

• Control add-in object: https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-control-addin-object
• Control add-in style guide: https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-control-addin-style

The style guide is particularly useful for ensuring that the add-ins you create match the look and feel of Business Central, not like the one we created here in this recipe (ha!).