Hi. I am in my first pregnancy (8 weeks, 1 day) and I have a random question that maybe someone can explain to me?

Been browsing through glow looking at other people's ultrasounds and at what point do the ultrasounds started becoming more of that curved shape? Is there a reason behind it? Is it different technology or different countries that just do things differently? (These are probably really dumb questions, but I'm young and curious)

For contrast, this was our first ultrasound at 7 weeks 3 days.