Complex file sharing permissions

Please could someone help with the following sharing scenario?

Root folder ‘A’ created by admin
1st level folders B1, B2, B3, B4 etc and their children created by admin
Admin has read write permissions over all

I want to assign read only permission to many people (a group) to folder A and all children
I also need to assign read write to Person1 using folder B1 and children, Person2 to B2 and children etc

How can I achiveve this please?

Go easy on me please - new to Nextcloud

Thank you