You can find Owners, Members and Visitors group and add users to these groups from Site permissions settings:
Go to SharePoint site and click on Settings (gear) icon
Select "Site permissions"
You can add users to groups from the Site permissions panel:
If you share the site with user and grant "Read" permissions, those will be added to Visitors group, if you grant "Edit" permissions, those will be added to Members group and if you grant "Full Control" permissions, those will be added to Owners group
If you want to see the groups and permissions, then you can navigate to classic experience site permissions page using "Advanced permissions settings" option from same panel:
OR use URL in this format to directly go to classic experience site permissions page:
By default Read permissions are assigned to Visitors group. So, you cannot prevent users with "Read" permissions from downloading the files.
There was a way in classic SharePoint server sites to prevent downloading files by granting "View only" or "Restricted Read" permissions to users. But, these permission levels are not available in communication sites by default.
You need to activate "SharePoint Server Enterprise Site Collection features" feature in your communicate site to get "View only" permission level.
Check this article for more information: Prevent Contributors from Downloading SharePoint Content