Literary devices are used to add meaning to words. There are many different kinds.
In line 35, Lady Macduff son enters. What she first says to him is use of verbal irony.
Father'd he is, and yet he's fatherless.
She is telling him that his father is dead, even though of course he has a father. What she literally means is that he looks just like his father, and seeing him you see his father.
The first words exchanged with them include a simile, which is basically a joke.
Sirrah, your father's dead.(35)
And what will you do now? How will you live?
As birds do, Mother.
What, with worms and flies?
With what I get, I mean; and so do they.
Her son saying that he will live “as birds do” is a simile, he is comparing how he will live to what birds do. He will get by where he can.
Lady Macduff picks up the simile and turns it into an extended metaphor. A metaphor is a comparison also, but it does not use the word “as” or “like” in the way a simile does. They continue to talk about his fate comparing him to a bird, and that is what makes it extended.
The conversation continues with the son asking what Lady Macduff will do for a husband.
Nay, how will you do for a husband?
Why, I can buy me twenty at any market.
Then you'll buy ’em to sell again.
Thou speak'st with all thy wit, and yet, i’ faith,
With wit enough for thee.(50)
Lady Macduff rebukes her son for making a pun about buying and selling husbands, but she starts it. Her son does not believe that his father is dead, and disagrees that he is a traitor.