We don't know.

There was no tribe called the Scotti that is known. Scotti was the term used in Latin to refer to Irish raiders who were attacking and in some places settling along the western coastline of Britain. These raiders called themselves Goidels which later became Gaels. The name of the Kingdom they created was Alba, but in Latin it was called Scotia.

Sometime around the 6th century AD the kingdom of Dal Riata was formed which roughly occupied modern Argyll in Scotland and Antrim in Ireland. It's possible that this kingdom existed before this though under a different name as there is no evidence for a cultural change either side of the Irish Sea, no invasion in other words. The Scots of Dal Riata were defeated a couple of times by the Picts and it's possible that the founder of Scotland, Kenneth MacAlpin was actually of Scots and Pictish ancestry. The best guess is that the Scots, either through taking the crown or through spreading Christianity or both, Gaelicised the Picts. Without knowing much about Pictish culture and language it's hard to gauge how close the two peoples were to each other culturally to begin with. All we know is that Scotland came into being in 843, possibly as an alliance against the Vikings who'd been raiding the coastline for the last century. People forget that large parts of Scotland were part of the Kingdom of Norway and that the Norse (Vikings) were there for hundreds of years. The Orkneys, the Shetlands owe more to Norse culture today, their original Pictish culture long gone.