@SauPom What types of customizations you want to make to 3rd party web parts?
You cannot make any changes in custom (3rd party) web parts directly from SharePoint modern experience.
You will need the actual code of web part to make modifications. Then you can re-package & deploy the solution on your site.
As you don't have the code with you, I would suggest you to reach out to support team who provided 3rd party web parts & request them for new features as per your requirements.
However, you can also develop new custom web parts on your own using SPFx. There are lot of samples available on GitHub repository which will help you to get started with.
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