Hi,
You can basically add the custom Elementor widget through a PHP file in your theme files.
there are some tutorials you can learn from online for example:
https://youtu.be/Ko9i153o_iU
You can also take a look at our guide here:
https://developers.elementor.com/creating-a-new-widget/
I hope this information helped you.
If you have any further questions, you may opt to visit our Github account where you may get in contact with a member of our development team to query this with.
Please note that this is not an official support channel but a respective member of the development team will respond to your post as soon as they are available.
Please follow this guide which outlines the steps required to post on our Github: https://github.com/pojome/elementor/blob/master/.github/CONTRIBUTING.md