How do I appear 'online' on facebook?

[dopaminedude]

Depending if I have the messenger app, mobile site "chat", etc. I see some people with green circles next to their name, and some with a little mobile phone. Does it change if I lock my phone?

 

[tekboi]

I've always been curious also. Not sure exactly how facebook determines if you're "online" or not.

But I think if you're on your desktop than you're online. But if you're on your phone than you're mobile?