The premium upgrade for Automate can be purchased through the Buy premium menu, also present as an option in settings. There’s no separate “pro” download, Premium is unlocked by a single in-app purchase which is owned to the Google account used during purchase. All devices registered with the purchasing account will have the Premium upgrade.
The free edition only has a single limitation, a maximum of 30 running blocks, that could be 15 running flows with two blocks, or a single running flow with 30 blocks. There’s no limit on inactive flows stored on the device. All features and blocks can be used with, or without, Premium purchase.