Struggling to make custom mobs spawn in structures.

Good day.

I am having trouble with trying to make my custom mobs spawn naturally in structures. I am currently making different versions of base game enemies, but I cannot seem to figure out how to, for example, make a custom monster mob spawn within a nether fortress.

I mainly use a wizard provided by Blockbench to make mobs and I use the bedrock vanilla pack as a guide to make most of my custom mobs' spawn rules files. So note that I have quite limited modding experience. So of course, it seems that mobs that spawn in structures are a bit more complicated to implement as the spawn rule files for them are basically empty. They only provide the population control component within their files and nothing else.

Any help would be appreciated!