How do you do a marketing for a mobile app?

Specifically, it might be more important to reach out to potential users through the niche channels that cater to whatever your app is about, i.e. through related sub-reddits, forums, group chats, etc. This also includes reaching out to specific individuals who you think would be interested and could spread the word to others due to their relatively loud voices - they might be people that regularly write reviews about new products in the space your app pertains to, or just people active enough in the target community that their word-of-mouth goes far.

The target market, which is an important concept in digital marketing, can be defined as “the people or groups your website/app wants to address”. In other words, you can show all the people you see as potential customers as a “target market”
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Definitely make a landing page, get some early beta users that you can really turn into advocates and start getting that WOM and UGC going