‘And Jacob went down into Egypt, and he died, himself and our fathers; and they were carried over to Shechem, and laid in the tomb that Abraham bought for a price in silver of the sons of Hamor in Shechem.'
So Joseph in Egypt was the source of their deliverance. And the final result of their deliverance was that they were buried in the land that God had promised them, in the tomb of their tribe. To those who had become obedient God fulfilled His promise.
Here we have another telescoped statement, presumably based on Jewish tradition with which his hearers would have had no quarrel. ‘Abraham bought' is on the basis that Jacob who did buy it (Joshua 24:32) could be seen as in the loins of Abraham (compare how in Genesis 25:23 whole nations are seen as in Rebekah's womb). We in our more pedantic way would say ‘the Abrahamic tribe, to whom the promises were made, bought'. It was important that it was connected with Abraham here, because it was to Abraham that the promises had been made (Acts 7:5).
Note that it is stated that ‘they' (our fathers) were carried there and laid in the tomb. We may assume from this that there was a Jewish tradition that most of the patriarchs were finally buried there (there were certainly Jewish traditions of the patriarchs being buried in Canaan), although the only information that we have from the Scriptures is of Joseph as being buried there (Joshua 24:32). Jacob was in fact buried with Abraham in Hebron (Genesis 50:13). It is therefore the other sons that are in question. But the important thing that Stephen was wanting to emphasise as concisely as possible was that the patriarchs had been finally buried in the land promised to Abraham. He simply selected a well known example in order to bring out the point.
Alternately Stephen may have seen Joseph's body as representing all their fathers, so that they were buried there in him symbolically. But if Joseph had made arrangements for his bones to be carried back to Canaan it is quite possible, even probable, that the others had as well, with the bones of Joseph getting special prominence because of his importance.
Some have seen the connection with Shechem, which in Stephen's time was connected with the Samaritans, as another indication of the ‘foreign' element so prominent in Stephen's speech, with the thought that even Jacob's sons were buried in a place despised by the present generation rather than in what they would see as the land proper.
‘Jacob went down into Egypt.' From Acts 7:9 onwards Stephen constantly mentions Egypt (thirteen times). He is thus stressing that until the time of Moses and for a large part of his life, Egypt was their focus and their environment.