In on of the extracts it says that black people were often forced to do ‘dirty jobs’;what does this mean and why were they the ones forced to do such jobs?

I assume you're talking about slavery and the slaves Simon Finch brought to his homestead to establish his farm.

So Simon, having forgotten his teacher’s dictum on the possession of human chattels, bought three slaves and with their aid established a homestead on the banks of the Alabama River some forty miles above Saint Stephens.