No, I don't think values according to which the branches are seperated are chosen at random. Instead, weighted average is calculated for each category and the category with the highest weighted average is chosen as the root node. This is also referred as Information gain
Consider this dataset
Consider the above picture, Here the outlook is chosen as Root node, And how is outlook chosen as root node?
First, We calculate the total entropy of the data. Lets say its 0.95. Now inorder the pick the right root node, We will find weighted averages of all the subcategories. There are 4 four categories here, So we will obtain 4 weighted entropy averages. Lets say they are 0.3, 0.2, 0.4, 0.8. Now we will subtract the induvidual weighted entropy averages from the total entropy. So we will get (0.95-0.3), (0.95-0.2), (0.95-0.4), (0.95-0.8). Among all the three which ever category has the highest value that category will be chosen as the root node. These 4 values are the information gain of each of the categories i.e Whichever category has the highest information gain, we will pick it as the root node. In our case, its the outlook category/feature .Hope it helps
Check this for more clarity