Attach carport with ground anchors - how to align?

Can the straps be attached either at position A or B, or does it have to be B?

Both are possible, there should also be some on the inside.

Definitely on the outside, so that the forces go directly into the first post. Otherwise, they first travel through the entire structure and into the post where the [Kopfband] goes down. Ideally both directions.

How long should such a headband be at least? I mean the distance to the point where purlins and posts come together.

Distance to the point where purlins and posts meet.

I would take 40cm +, wind and weather, you put it on top, it should last 50 years. And don't use small screws for fastening, something like 6x120 and at position right A around 5pm.

And don't use small screws for fastening, like 6x120 and at about 5pm on the right A

isn't the beam only 90x90? Gluing and several small screws will probably help more