Google & SEO for us.
The best way to find a good channel is to try to promote your service in many different channel, and see which one performs best, and then focus more time on that specific channel. It really differs depending on the product, for some Linkedin is amazing, for others, Twitter gives a better ROI.
The biggest lesson that I've learnt is that only you know who your customers are and how to reach them. This means that what works for one person has no bearing on whether it will work for you.
Start with a plan and more importantly sequence (order); ASO 1st, then SEO etc. Tackling one channel at a time allows you to determine how effective it is and whether your message is working (it will most likely need adjusting) - too much too quick becomes really difficult to measure. Run each channel as an experiment and measure it.
All channels are inherently different in some way as some entrepreneur has come up with an idea slightly different to what's out there and they have created a way to monetize it. This means they have different rules and varying feeding requirements. Some are quick time to value and others are a much slower burn, some need you to contribute and earn the right and others are just about spending money.
Lastly, if you're in a regulated space like me, it's much harder and one that no one on the internet talks about or understands :)
Indie hacker communities could work well for you. Try to join LUNADIO. https://www.indiehackers.com/ will be nice too, but it is invite only now.
Otherwise, Quora could work well for the type of product.
Linkedin outreach is also an option.