I'm still pretty new to JSS/Sitecore and am trying to wrap my head around some of the concepts. One thing I don't understand is if its possible to create new layouts/templates on an existing app and deploy just the content of the new layouts/templates while not overwriting the existing content on the old layouts/templates.
By the default, deploy will not override templates or layouts imported previously (unless used with
-w option). You can also try:
jss deploy template- can be used when you want to scaffolds out a new template in Sitecore or modify existing one (e.g.: to add fields or change field types)
jss deploy component- scaffolds out a new component (rendering item, datasource template, and allowed placeholder settings) in Sitecore
For more details take look here: https://jss.sitecore.com/docs/fundamentals/cli