Hi,
I used to create Nintex Form and Workflows (Site Collection re Usable) in SharePoint onprem . I don't see this features in online. I am in the process of migrating from onprem to online. We create the forms and workflows in the site collection level and use a site template to create sites which will inherit all this forms and workflows. The advantage is if I need to update anything is form or workflow , I make the change in the site collection level and the changes are trickled down to the subsites.