Hello,
For Elastic Agent it depends on the output, if the output is elasticsearch and you have multiple hosts configured, then it will use round robin to load balance between them, if the output is logstash you need to configure loadbalance: true for it to load balance between the nodes.
The documentation is here and here.
For Logstash if you have multiple nodes in the hosts settings it will load balance between them using round robin as explained here.
And for Kibana is the same thing, if the setting has multiple nodes it will load balance between them, the documentation is here.