Explanation:
Box 1: Create a new team, and the business unit users, and the assign the security role to the team.
Change the business unit for a team
Important
By changing the business unit for a team, you can remove all security role assignments for the team. At least one security role must be assigned to the team in the new business unit.
* Select an environment and go to Settings > Users + permissions > Teams.
* Select the checkbox for a team name.
* Screenshot selecting a team.
* On the menu bar, select Change Business Unit.
* In the Change Business Unit dialog box, select a business unit. Enable Move records to new business unit to move to a new business unit. Select OK.
Box 2: Grant the user a security role from the child business unit.
Reference: https://docs.microsoft.com/en-us/power-platform/admin/create-edit-business-units
https://docs.microsoft.com/en-us/power-platform/admin/security-roles-privileges