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
.
TLDR;
The modern ELBs use round robin routing algorithm
for HTTP/HTTPS requests
and 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