Just like Ganesh said, if you need to only share the particular list to the users. Break the permission inheritance for that list and share the list to the users.
And you can get more information about permission levels inheritance here. Check the "Overview and permissions inheritance part"
Understanding permission levels in SharePoint
If you want to add users to any particular SharePoint Online list only the you need to break permission inheritance on list and grant then grant the access to user on that particular list.
Reference: Search for "Break permission inheritance in SharePoint Online" on below article.
Customize permissions for a SharePoint list or library.
Solution for 1st question:
In SharePoint Online Modern Communication Sites, you can create a main menu with Nested links & sub-links and display the hierarchy in the form of a sitemap like shown in below image:
In SharePoint Online you can create either Header (without link) or URL for navigation to another pages/documents in your site. So you can ...