It depends on the type of ELB used. AWS has over the time introduced Application ELB and Network ELB along with the Classic ELB.
Application Load Balancers applies listener rules and assigns the (HTTP/HTTPS) request to a target group. It selects a target from that target group using the
round robin routing algorithm
Network Load Balancers node that receives the connection, selects a target from its target group using a
flow hash routing algorithm
Classic Load Balancers uses
round robin routing algorithm for TCP listeners and
least outstanding requests routing algorithm for HTTP and HTTPS listeners
The modern ELBs use
round robin routing algorithm for HTTP/HTTPS requests
flow hash routing algorithm for TCP requests.
Classic ELB used
round robin routing algorithm for TCP requests and
least outstanding requests routing algorithm for HTTP and HTTPS requests
Source / Further Reading : https://docs.aws.amazon.com/elasticloadbalancing/latest/userguide/how-elastic-load-balancing-works.html#routing-algorithm