From what I understand, the Scots language is derived from Old English.
From my point of view, the Lowlands of Scotland are like England, in the sense that they are a mix of Germanic and Celtic.
The English people are descended from Angles, Saxons and Jutes who assimilated and absorbed the Celtic people and land of England.
Isn’t this the same case with the Lowland Scots? They absorbed the Celtic land and people of Lowland Scotland?
Both English and Scots are Anglic languages.
In contrast, Scottish Gaelic is a a Celtic language, brought over by an Irish tribe called the Scotti who assimilated most the Pictish Highlands.
In Scottish Gaelic, the word for the Highlands translates to Gaidhealtachd, which means the Gaeldom. This ‘Gaeldom’ doesn’t include the traditionally Scots speaking Lowlands.
To me, it seems like the Scottish Lowlands and England are Anglic, with the Scottish Highlands (+ The Western Isles, which is usually included in the term Gaidhealtachd), Ireland, Cornwall, Isle of Man, Brittany and Wales being Celtic.
Can someone try and explain the situation better to me?
There is an error in the way you describe the background of your question.
the Scots language is derived from Old English.
That is correct.
The English people are descended from Angles, Saxons and Jutes who assimilated and absorbed the Celtic people and land of England.
That is basically correct, although I'm not comfortable with the terms "assimilated and absorbed." They linguistically conquered the Celtic lands that would later be known as England, but the indigenous population retained a great deal of integrity in many places and in many ways. I would not describe this as "absorption" but that's a matter of terms, and I get your point.
Isn’t this the same case with the Lowland Scots? They absorbed the Celtic land and people of Lowland Scotland?
This is where an error is implied. By "they absorbed," it appears that we are talking about Anglo-Saxons "assimilating and absorbing" southern Caledonia in the same way that they did, say, the southeast coast of Britain (where they apparently arrived in sufficient numbers to have a real effect on the indigenous population).
To be linguistically conquered does not imply military conquest, nor does it imply any sort of "genetic" dominance. Language spreads aside from political control and/or the arrival of a conquering population. The million of Irish who over the centuries were coerced into abandoning their native language, surrendering to English, did not cease to be Irish.
In England, some case can be made that not only was there a linguistic conversion, but a group of new people came to join the indigenous Celtic population. Some of that sort of thing has occurred throughout Britain and Ireland - people have always been very generous with their genetic information - but there is a world of difference that separates the Celtic "nations" of Britain and Ireland from what became known as England.
Caledonia was enormously complex, composed or Gaelic speakers, Brittonic speakers, perhaps a pre-Celtic language that survived for a while (that is not clear nor is it widely accepted), and even some Scandinavian presence. In addition, as you indicate, some people underwent a linguistic conversion that imported a form of Old English. It is also not clear that Scots Gaelic was "brought over by an Irish tribe called the Scotti who assimilated most the Pictish Highlands" as you say: it has also been argued that western Caledonia underwent a linguistic conversion, switching from their Brittonic language to the dominate language spoken in the nearby, powerful area of Ulster in Ireland. That is being debated. And again, genetics have a way of being exchanged, and people move here and there, but we may not be looking at an actual process of conquering and assimilation.
It is quirk of history that Caledonia became known as Scotland, honoring only one of the languages spoken in Northern Britain. Of all the Celtic "nations" Scotland is the most complex. It was a bit of a "stew" of people, but these various people for the most part did not endure the sort of colonization that typified England. They were able to resist the encroachments of the Anglo-Saxon people even if some of them found it reasonable to adopt their language.