How to add Custom Field on Topics by category

I would like to understand how to create a custom field for topics depending on category (I’d like to be able to select which categories this Custom Field will be visible.

Ideally I would like 2 custom fields

1 Text field
1 image upload field

The custom fields should display on the layout sidebar display box,

However i’d like text field value ( added when creating a topic and will be a URL link to an MP3 file) to display a player to play the MP3 file

The image custom field upload will just be a logo to display above the Text field

If anyone can help point me in the right direction I would be grateful

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