How many of you have noticed that there is a custom tab while creating site collection? What is it purpose ? Let me explain it here.
- Save the site which you want it to be a custom template for your site collection.
- New a site collection in SharePoint Online, and choose Select template later under Custom tab.
- Open the new site collection, then click Solution Gallery to upload the template and activate it.
Thus SharePoint enables you to use custom template to create site collection apart the list of OOTB templates.