[SalesForce] How to give a permissions of business units in marketing cloud

You can use the REST API to retrieve an existing Interaction using the getInteractionById method, then using the payload from the response, use the createInteraction method to create a new one.

However, the problem is that activities are specific to the Business Unit. For example, emails relate to a Triggered Send in the Business Unit, filters used in Decision Split Activities and Events relate to filters in the Business Unit, Update Contact Data Activities relate to Data Extensions in the Business Unit and so on.

So yes, you can copy an Interaction using the REST API, but you would need to re-configure the Interaction Event and its Activities after doing so.

Not sure I get the full scope of your question, but overall - yes. All subscribers added in the Child Business Unit will also be added to the All Subscriber List in the Parent Business Unit - and thus, if you send an email to the all subscriber list - yes, you would have access.

If your data model is based on data extensions, then this wouldn't be possible from the Parent, given that the parent doesn't have access to data extensions stored in the child business unit.

However, if you use data extensions, you can populate a "total audience" data extension in the child business unit, by selecting data from both the child and the parent bu (i recommend 2 queries), first one selecting data from child, second selecting from the parent by adding ent. in front of the data extension name.

All depends on what you are trying to achieve.