We can apply label on list and library level. Is there any way we can apply label on site collection level so it will be apply to all list and library inside root site as well as in sub sites?
Yes, this is possible but you need have that SharePoint site selected (Communication site) or Group (If it's a Teamsite) within Purview, This can be done by Adaptive Scope, so if tagged a sensetivity label/URL/Property bag or Static where you add the specific URL
