Junos works as per flow load blancing, and different platform might use different method to approach this.
Eg, you can check below for legacy EX
https://kb.juniper.net/InfoCenter/index?page=content&id=KB22943
and below for QFX5100 and EX4300 for the details.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28946
Regards,
Alex Wei