Reference Material:

https://docs.aws.amazon.com/vpn/latest/s2svpn/VPNRoutingTypes.html#vpn-route-priority
When a virtual private gateway receives routing information, it uses path selection to determine how to route traffic. Longest prefix match applies. If the prefixes are the same, then the virtual private gateway prioritizes routes as follows, from most preferred to least preferred:
BGP propagated routes from an AWS Direct Connect connection
Manually added static routes for a Site-to-Site VPN connection
BGP propagated routes from a Site-to-Site VPN connection
For matching prefixes where each Site-to-Site VPN connection uses BGP, the AS PATH is compared and the prefix with the shortest AS PATH is preferred.