There is no need to login to other instances. Just browse them, and if you want to comment something copy the link and paste it in threads search (Lemmy) or microblog (mastadon) and you can interact with them. I hope in the future apps will make browsing much easier and seamless without copying and pasting links around.
There are browser add-ons for the desktop that help with that process.
1000+ hours, Port Royal 2, over the last 6+ years. I just love expanding my trade empire.