Hi @NotFranmax - Are you using On-Prem orchestrator? Which version it is? If it is On-Prem, user-machine mapping is available only from 21.4 onwards.
You cannot achieve " Robot rA should work only on machine A, robot rB only on machine B" with Triggers in single modern folder if you are using 20.10 or lower versions. You will have to create two different modern folders and assign one machine to each of them. So that when you create triggers for Robot rA, it will find only machine A to run.