Presumably, through social interaction with others in the communities they are a part of. That's how I heard about OpenTTD in the early 00s, at least.

This question tickles me. In the before time, something would be so good you were compelled to tell someone about it.

Sriracha, Costco are brands you likely know that dont advertise, and somehow got popular. In the 90's there were bands that were massively popular with little to no air play, and less promotion (Fugazi is a great example).

You're asking a leading question. The verb you're using here is one specifically indicating interaction with a "platform" (a digital aggregation of information). The answer is you don't search anything, you completely change your epistemic and interaction model. Instead you build a social web of people who have their own social webs, and you share things you've made and things that have been shared with you. This is your "platform".

How are most games on steam found? I kinda doubt all people find them through steam own mechanisms. I even doubt the majority find them this way. Gaming has multiple sources of information, be it news, social media, influencers or cooperations. Video-content is probably the biggest source of being discovered for most games these days.

It's not like you can discover it on Steam any easier.

Of course, searching for information itself is also a skill, but it is a truly essential one for the modern world.