When podcasters talk about "Finding your niche" I think they are sometimes talking at cross purposes. We come from a really wide range of backgrounds, professional, demographic, geographic, economic etc. What makes something a podcast, what you want to achieve with it and how you want or need to find your audience are all going to have different answers depending on this. It will also depend on what you really mean by "niche".
To my mind we can mean one of a few different things when we say niche:
Audience. Narrowing down the question of "Who wants to listen to this?" is an important one. To broadcasters this often means age or gender, and sometimes broad political pursuasion. It might mean someone who listens to other shows. It will out of necessity be a large "niche"; if you walked down a street you would see people who could be a fit. To a podcaster it doesn't need to mean this; it can mean someone with a specific hobby or interest. This might make the audience easier to find if there are already communities around this activity.
Scope. The style or topics you'll be covering. You could make shows about a small area for a general audience, or for a specific audience. Provided your output is good there is definitely room in podcasting for one feed which has different episode styles within it, provided the subject matter is consistent. E.g. You can have long form stories, interviews, news pieces all as separate parts. Especially if some content is evergreen and some is time sensitive.
Perhaps the most important part in podcasting is that you need to find a balance between these two.
If you make a show which has no particular focus, and no audience in mind you aren't going to attract or retain many listeners. If you have a general audience but a well defined style you stand a chance - but take a look at the landscape for this. Shows in this category are already well contested for audiences, attracting those first listeners to help you grow is going to be difficult. Nobody is going to look at your show and see a part of themselves in it, they won't be invested. These shows tend to work if they're established by people with an existing audience, and even then they need to be done well. Shows in this category would be interviews with celebrities, or the stereotypical three white dudes shooting the breeze, it's hard to have a hook. Maybe your material is great, but I think you have to be building audiences elsewhere.
The other angle is that your show could have a very niche audience in mind. If it's for or about a particular community this will help with finding that audience, and it makes the target audience more likely to want to listen. Obviously it limits the potential size of your audience but that might not be a problem. It really depends on what your goal for your podcast is; for many people it's promoting their business, or themselves as a leader in their industry. You don't need a huge audience for that to be effective as long as it has a high impact on the audience you do have. A show produced for a small audience like this can be great because you talk to them in their own language, it forms a community around it.
All the way through this is the idea of finding your own voice and your audience, and how the two are symbiotic. Podcasters are always asking how to build their audience and it needs these two factors; you need to know who they are and you need to speak to or for them in a voice they understand. That means online adverts, social media and so on can be useful, certainly in retaining a community around your work, but finding that audience, getting them to hit play enough to become invested, getting them to share your work with others, it's hard. I think podcasters need to think more about being there physically. Think live shows or events, promotion in person. If you can identify the places that will be productive then you'll find an audience there to work with.