Site Template for existing Site Collection

[Sajith G H]

We have default (Microsoft out of the box) SharePoint  online site collections for different departments.  The  organization want to create a template (Site Template) for these Site Collections using customization in  modern team site. Can we use the custom site template for the existing Site Collections for changing the layout according to the organization ?

The existing Site Collections templates are Modern Team Site with No Office Group, Team Site Classic Experience.

[#2]

What you can do is apply PnP Templates or Site Scripts & Site Designs to your existing sites so they follow the same customization