How do you do a marketing for a mobile app?

For mobile apps I’d start with exact-intent communities, not generic launch channels. I’m doing this for Kinetic Override by looking for people already asking about no-root Android auto clickers, macro recorders, repetitive testing loops, or idle-game tap fatigue. The copy becomes much easier when the thread already names the pain.

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