Is it possible to have custom, default fields added to all new product types In Commerce 2? I have multiple custom fields that will need to be on all new product types (Commerce 2). The fields are already created on the 'default' product type. But new product types do not inherit the custom fields.

Is there a way to have custom fields automatically added to all new product types?
Nothing that just works.

You could define them as a base field through code by defining them in hook_entity_base_field_info(), so for example how scheduler module does that. Then they will be non-configurable fields that are always there. If they're not multiple, they will even be added to the product base tables.

Those kind of fields are a bit tricky to uninstall.

The other option is implementing a hook_commerce_product_type_insert() and then copy the configurable fields you want from the default type and add them to that new bundle as well.