I just spent many hours on this issue myself and I’m convinced that the docs are vague to force you to ask for help, at which point they say it’s complicated and refer you to the certified “experts”.
Maybe this will point you and others in the right direction. Look at the variant object for the the property “admin_graphql_api_id” that is a string starting with “gid://shopify…” and base64encode that using the JS function btoa( passing in the string ) and try adding the result to the checkout as the “variantId”
Not happy about the intentional ambiguity at all