(I don't know what to say about this other than this is the basis for a fic in headcanon form)
They meet when Zelda brings Terra to get inspected (during Road to the Ancient lab)
At first Terrako was kind of put off as Robbie was poking around inside it’s head.
Terrako then beeped something akin too 'you're really loud.' And Robbie apologised and tried to quiet his voice.
Terrako was shocked, beeping super loudly and rather annoyingly.
"And you’re saying I’m too loud? What do you mean you can understand me? 'Couse I can. I can speak guardian."
After Terrako realised Robbie could understand it, it became obsessed with talking to him.
Robbie loved talking to the little egg. Not many guardians had the sentience to hold full conversations. It was either Terra or the Devine Beasts, who’s dialect was slightly different so Robbie had a hard time deciphering it.
Over time Robbie realised that Terrako was a a bit of an asshole and he loved it for that. It complained a helluva lot about the other champions. Saying that Revali was rude, Impa bullied it, Link was a show off, Urbosa didn't like it and Daruk didn’t really care. It seemed the only people it like were the princesses. It thought Mipha was just really sweet and Zelda was perfect.
Robbie realised that this guardian, it was pretty much a child. He started treating it like one.
Although he soon came to the conclusion that he was really the only one who treated Terrako the way it should be treated. No one was being horrible to it but it needed to be treated like a child, not a war-machine.
Robbie called an official meeting amongst himself, Purah and the Champions (bar Terrako). He explained to the lot of them, he can speak Guardian and Terrako wasn’t what everyone thought it was. It was just a kid and it needs to be treated like one.
Purah (who also learnt some Guardian communication) vouched for Robbie.
Robbie understood not everyone could hold a conversation with it but as far as he was aware, everyone could make the effort to treat it right.
After the conference, Zelda stayed behind to request lessons on Guardian communication from Robbie. Who, after warning her it takes years to understand, of course accepted.
Link started trying to talk to Terrako, realising the nagging voices in the back of their head was in fact the Robot and they in fact can also understand guardians
They blame Hylia for that.
Robbie and Terrako? Inseparable bartards from this point on. If Zelda has a conference Terra can’t attend, Robbie babysits it for the weekend.
Robbie fixes it up a bit, him and Purah makes fun of Terra’s little child crush on princess Mipha, they listen to music.
And they see eachother every couple of weeks just to hang out. Maybe go sparing
Basically Terrako now has a dad
(@redshift-corridor feel like this is something we'd probably talk about so tagging you lol)