How to add existing product variations to a new product?
The option you're looking for is not present on purpose (and the widget itself is different despite how it worked in Drupal 7). Every product has its own variations that cannot be shared, so there's no way to reference the same variation from multiple products.

If you need to sell the same thing from multiple contexts, what you probably need is some other entity that references the same product, not a standalone variation. Products:variations have a very clear parent:child relationship ... they're "variations" of the main "product".