I have no experience in this other than the fact that I have done some research regarding the launch of a iPhone app for a previous employer. But seeing as no-one has answered yet I thought I'd point you to this: http://advertising.apple.com/promote/

I know nothing about the effectiveness of this platform, but from what I understand Apple will advertise your app to users of other apps. Hopefully that's at least somewhat helpful. :)

Paid Ads on social networks can generate some downloads however engagement for continuous usage of apps is critical to success of your venture. Ads can't buy engagements unless Inapp features are creative or outside app campaigns (instead of ads) are gamified in a indirectly relevant mechanism.

Prelaunch gamification will certainly be very helpful in your case.
I have found that if you reward prospective users of your app using indirectly relevant gamification, users get engaged with your venture. The most activity mobile app user does after texting and phone calls is Take Photos.
I would suggest to create a gamified campaign around taking photos and reward those prospective users for taking 3-5 photos, social sharing etc.
Reward something simpler like what they can physically take home or share or use while traveling etc.
now once the campaign is over, notify all the participants to social share and also introduce your actual app. There will be a great chance of these users becoming your app's genuine users and continue using your product.

We solve these problems with www.oly.io and better gamification under startups market interest discovery www.mobisoftinfotech.com.

Good luck!
Best,
Shail

I think survey frameworks in apps are the most revolutionary and engaging

But also services like Kipp are great!

Because of their broad audiences and ability to segment, Facebook and Instagram are very effective advertisement platforms for mobile apps. And since Facebook owns Instagram, they make it incredibly easy to places app install ads on both.
You can read more here: https://uplandsoftware.com/localytics/resources/blog/8-ways-to-promote-your-app/#:~:text=Because%20of%20their%20broad%20audiences,app%20install%20ads%20on%20both.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath