The link you click on is called a Universal Link and it allows the web developer to create a link that opens the associated app automatically, even if not installed.
When a user is browsing your website in Safari and they tap a universal link to a URL in the same domain as the current webpage, iOS respects the user’s most likely intent and opens the link in Safari. If the user taps a universal link to a URL in a different domain, iOS opens the link in your app.
What this means is if you’re already on the Reddit site for example, if you click a link, it won’t launch the app. However, if you click on a link from somewhere else, like a web search, it will launch the app. This is the default behavior in iOS 9 and up.
While the documentation references Safari, Chrome and Firefox both respect the universal links.
You can “bypass” this In Safari on a per site basis by long pressing the link then selecting “Open in Safari”. Every link after that will open in Safari. You can reset it by long pressing the link again and selecting “open in app”.
Because Chrome and Firefox respect the link, once you set it in Safari, the other browsers will follow suit.