Answer by taking a quote from the following article:

The Scottish people (Scots: Scots Fowk, Scottish Gaelic: Albannaich), or Scots, are a nation and ethnic group native to Scotland. Historically, they emerged from an amalgamation of two Celtic-speaking peoples, the Picts and Gaels, who founded the Kingdom of Scotland (or Alba) in the 9th century. Later, the neighbouring Celtic-speaking Cumbrians, as well as Germanic-speaking Anglo-Saxons and Norse, were incorporated into the Scottish nation. In modern usage, "Scottish people" or "Scots" is used to refer to anyone whose linguistic, cultural, family ancestral or genetic origins are from Scotland.

In the Early Middle Ages, Scotland saw several ethnic or cultural groups mentioned in contemporary sources, namely the Picts, the Gaels, the Britons, and the Angles, with the latter settling in the southeast of the country. Culturally, these peoples are grouped according to language. Most of Scotland until the 13th century spoke Celtic languages and these included, at least initially, the Britons, as well as the Gaels and the Picts. Germanic peoples included the Angles of Northumbria, who settled in south-eastern Scotland in the region between the Firth of Forth to the north and the River Tweed to the south. They also occupied the south-west of Scotland up to and including the Plain of Kyle and their language, Old English, was the earliest form of the language which eventually became known as Scots.  Use of the Gaelic language spread throughout nearly the whole of Scotland by the 9th century, reaching a peak in the 11th to 13th centuries, but was never the language of the south-east of the country. King Edgar divided the Kingdom of Northumbria between Scotland and England; at least, most medieval historians now accept the 'gift' by Edgar, in any case, after the later Battle of Carham the Scottish kingdom encompassed many English people, with even more quite possibly arriving after the Norman invasion of England in 1066. South-east of the Firth of Forth, then in Lothian and the Borders (OE: Lodene), a northern variety of Old English, also known as Early Scots, was spoken.  As a result of David I, King of Scots' return from exile in England in 1113, ultimately to assume the throne in 1124 with the help of Norman military force, David invited Norman families from France and England to settle in lands he granted them to spread a ruling class loyal to him. This Davidian Revolution, as many historians call it, brought a European style of feudalism to Scotland along with an influx of people of Norman descent - by invitation, unlike England where it was by conquest. To this day, many of the common family names of Scotland can trace ancestry to Normans from this period, such as the Stewarts, the Bruces, the Hamiltons, the Wallaces, the Melvilles, some Browns and many others.  The Northern Isles and some parts of Caithness were Norn-speaking (the west of Caithness was Gaelic-speaking into the 20th Century, as were some small communities in parts of the Central Highlands). From 1200 to 1500 the Early Scots language spread across the lowland parts of Scotland between Galloway and the Highland line, being used by Barbour in his historical epic The Brus in the late 14th century in Aberdeen.  From 1500 on, Scotland was commonly divided by language into two groups of people, Gaelic-speaking "Highlanders" (the language formerly called Scottis by English speakers and known by many Lowlanders in the 18th century as "Irish") and the Inglis-speaking "Lowlanders" (a language later to be called Scots). Today, immigrants have brought other languages, but almost every adult throughout Scotland is fluent in the English language.

Was there conflict among the Picts, Gaels, Britons, and Angles in Scotland?
European style of feudalism to Scotland along with an influx of people of Norman descent -