Major differences between breath meditation and body scan meditation? I have done several Vipassana retreats in the past. Some questions came up to me several times without being able to answer them.

What are the major differences between mindfulness of the body (e.g. Body scan/Vipassana meditation) and focusing on the breath (i.e Anapana Sati meditation)?

How can I focus my attention as it arises across my body? I am able to focus on my breath between my nose and my mouth, but when I am aware of my sensation on my legs for instance, should I stay aware of my breathing simultaneously?
For starters i am not sure how you differentiate between the two. The literal content of anapanasati sutta teaches us to examine several different aspects including but not limited to bodily sensations or breathing.
https://www.accesstoinsight.org/tipitaka/mn/mn.118.than.html

What are the major differences between mindfulness of the body (e.g.
Body scan/Vipassana meditation) and focusing on the breath (i.e
Anapana Sati meditation)?

Besides the obvious difference in meditation objects, there will likely be a wider range of experiences perceived during a body scan, as you become more receptive to any/all body functions, aches, hunger, et c, compared to focusing solely on the breath.
Also, scanning the body systematically too quickly can make you less perceptive to anicca - i.e. the non-permanence of a singular sensation - compared to being attentive to your breath only where you'll be gradually more attentive to subtle changes. On the other hand, shifting your attention as taught in anapanasati can aid you in practicing the discernment of mental factors.

How can I focus my attention as it arises across my body? I am able to
focus on my breath between my nose and my mouth, but when I am aware
of my sensation on my legs for instance, should I stay aware of my
breathing simultaneously?

Choose one object at a time. It will train you in maintaining singular focus (ekagatta), and will also enable you to examine how bodily phenomena transforms, or whether they are successively stilled (or not), for instance. If other sensations arises you will likely shift your attention regardless of whether you intend to or not, so focus on practicing returning your attention to your initial meditation object.