Earlier quoted context omitted.
Usually they are websites. That's kinda the whole point... Spotify's desktop app is the same as open.spotify.com, Discord's is the same as discord.com, VS Code's is the same as vscode.dev, Slack is the same as.. something. But yeah, instead of those companies all needing 5 separate engineering teams (Web, Mac, Windows, iOS, Android, and you're joking if you think they'll make a Linux native app), they just make a web…
I think it's a huge pity that we're so reliant on platforms instead of protocols - there would be native Slack clients for every platform. Few companies even allow proper API access. Spotify buck the trend, Spot is a fantastic app https://github.com/xou816/spot
We need regulations around platforms, protocols are inherently more powerful and from the perspective of humanity almost always preferable over platforms, so the incentives of companies should reflect that, but it isn't right now.
Stuff like email, http etc is what makes computers great. I doubt anything like that could get invented today, companies would just create their own proprietary protocols and there would be no web browsers. Imagine if we just continued along that route and made protocols for everything instead of having big companies lock down computing habits via platforms.