Resource groups do not incurr any cost unless you add resources to it. Its just a placeholder.
Sandboxes provided by Microsoft Learn platform on Azure are totally free, you can spin as many resources, resources groups as you want based on your needs and exercise requirements. Microsoft will just put limits on the size of VM's - you will not be able to go beyond general compute category VM's, services like CosmosDB via policies on the subscription.
Hence feel free to play and experiment.
Hopt this helps.