RI TRANSFER under same root account

yes, that's right; by default, RI sharing is enabled in AWS Organizations - https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/ri-turn-off.html

so, if you have few accounts linked in AWS Organizations, and if you buy reserved instance or savings plan on one of them, that account will just have a priority over other accounts to benefit from the discount; but if you shut down resources on that account - the discount will roll over to the rest of Organization provided that other accounts run resources that have matching attributes to those of RI