ID and Legacy ID would be the same, the difference is that ID is base64 encoded and Legacy ID is the one displayed on the UI of the app. The plan was to deprecate the Legacy IDs and use the UUIDs which are more secure, but we do not have plans for doing so in the near future.
Partner Order ID is usually the ID from the store. For example if its a Shopify Order this will match the Shopify Order ID. When creating an order you can specify this to whatever you want as long as its not repeated on your account.
Same in this case for ID and Legacy ID and Partner ID. The Order ID should be the Legacy ID for the order.
Please let us know if we could help and provide a better explanation about this.
Thanks in advance!