Long answer short, yes they do! While creating your DMARC record if you leave the sp attribute blank then your DMARC policy for your main domain will be inherited by your subdomains, like in the case shown below:
Alternatively, you can choose to enable a different DMARC policy for your subdomains. In that case, you need only mention the policy mode you want to configure for them and it will be highlighted in the sp attribute of your DMARC record, as shown below:
Note: If you don’t want to configure a different policy mode for your subdomains, you don’t need to enable it. It can be left default in that case.