Three blogs in one?

I’m still very much a beginner with WordPress, so apologies if I’m missing something obvious, but can anyone tell me whether it’s possible to split a site so that the categories list changes depending on which section of the site someone’s in?

It’s probably easier to show than explain. Here’s the site I’m working on: https://then-now.org.uk.

I want to be able to show then and now photos of both places and people. But the categories in the sidebar of each section would need to be different. Just place names in the places section, and people’s names in the people’s section.

Presumably one way of doing this is to install the same customised design of WP in two sub-directories: /places and /people. Plus another one at the top level to bring them both together. But before going down what seems an over-complicated route, I just wondered whether there’s an easier way of doing this.

Any help or advice would be much appreciated.

Thanks