I have difficulty understanding what a word in the end of a sentence refers to.
Firms can gain a better understanding of their purchased supplies and their journey.
Does 'their journey' in this sentence refer to the purchased supplies (as intended) or to firms? How should it be rewritten if it is incorrect?
Your quotation is a good example of ambiguity. There is nothing wrong with the sentence, it just isn't clear from the grammatical structure what the second 'their' refers back to. Careful writers try to avoid ambiguity, but we often just have to use context and logic to determine what is meant.
My old English teacher's favourite example of ambiguity was "my father drives to work in a hat". It could be read that the hat is his vehicle, but obviously, that is absurd and so it cannot mean that. That is an example of using logic to dispel ambiguity.
With your example, it seems unlikely that it refers to a 'journey' made by "the firms". It immediately felt obvious to me that it was talking about the 'journey' made by the goods they order. Perhaps the wider context confirms that - for example, if it was from an article about 'carbon footprints' made by businesses, it would seem logical that the distance goods travel would be under consideration.
Sometimes you can tell what a pronoun refers to by the number and/or sex.
Example 1: "Bob and Sally drove to the market in his car." "His" here must refer to Bob and not Sally because "his" is masculine, Bob is masculine, and Sally is feminine. So "his" matches the gender of Bob but not of Sally. (One could quibble that I am assuming that "Bob" is a man and "Sally" is a woman. It is, of course, possible that Sally is a man. But not very likely, and unless in context we are told that Sally is a man, readers would normally assume that she is a woman.)
Example 2: "The people in the crowd booed when the actor gave his speech. Then they threw rocks." In the second sentence, "they" must refer to "the people in the crowd" because it is plural and "the people" is plural. It can't be the actor who threw rocks because "they" is plural but "actor" is singular.
Besides that, you have to rely on logic and context. Like suppose I read, "Bob and the dog entered the house. 'Hello', he said." I'd normally assume that "he said" refers to Bob and not the dog, because dogs can't talk. (If this sentence occurred in a story about a talking dog, then it would be unclear.)
People sometimes -- often -- say or write sentences that are unclear because such references are ambiguous. When the person is saying the sentence, he knows that when he says "he" he means the tall man with the red hat (or whomever), but he fails to make that clear to the listener.
