It's not recommended to use soap directly. CSOM offered TermStore related class to manage and obtain information from the Termstore in Sharepoint Online.
To get terms via CSOM, you may take a reference of below blogs:
It is not possible to set alerts in custom scope if you plan to use the OOB search alert function.
SharePoint only provides three change types to set alert on and no other options are available.
You will have to look for custom solutions like event receivers.
Another reference: SharePoint 2013 Search Alerts, Programmatically!
It seems audience targeting on navigational links will be the thing you want. The site owner must enable audience targeting for site navigation. Once enabled, any site editor can target menu links to specific audiences.
You can check it here:https://support.microsoft.com/en-us/office/target-navigation-news-and-files-to-specific-audiences-33d84cb6-14ed-4e53-...