Hey Ronald,
A good reference for constructing your search queries can be found here: Search reference—ArcGIS REST API: Users, groups, and content | ArcGIS for Developers
From that reference you will note that if we're we searching for items and want to return items form a particular group, we need to specify that group by it's ID instead of the group name or title. You could grab the group ID by browsing to the group in the browser and copy id parameter from the address bar of your web browser. Alternatively - if we wanted to do it all through code we could search for the group by title, and utilise the id property of the matching group object in our query that's searching the content.
group_name = "My Group Name"
group = gis.groups.search(query="title: {}".format(group_name))[0]
items = gis.content.search(query='type:"Feature Service", group:{}'.format(group.id))
Going back to that link I provided, you will also note that we don't have the option to restrict a search to a particular folder. We do however have an ownerFolder property on items that are returned from a search which specifies the folder an item resides in by its Id. So another we could approach this would be to firstly work out the ID of the folder we're interested in - again this could be done through the web browser, or via code - then get a list of all the Feature Service items that belong to a user and filter on that list using each items ownerFolder property.
username = ""
target_folder = ""
user = gis.users.search(query=username)[0]
folder_id = [folder['id'] for folder in user.folders if folder['title']==target_folder][0]
user_items = gis.content.search(query='type:"Feature Service", owner:{}'.format(username))
items = [item for item in user_items if item.ownerFolder == folder_id]
Hope this helps!
