Topics that are created by the internal Salesforce users are separate from the Topics that are created per each community under Content Management –> Topics. External/Guest user cannot access the topics that are created by the internal users. But, they can access the ones that are created in their Experience Cloud Content Management.

Salesforce Idea
https://trailblazer.salesforce.com/ideaView?id=08730000000lBGQAA2

SOQL:

SELECT Id, Name, NetworkId 
FROM Topic

Note:

NetworkId is the identifier of the Experience Cloud site to which the topic belongs. This field is available only if digital experiences is enabled in your org. If NetworkId is blank/null, then they are created by internal users.