Hello Satush and welcome to CiC. Thank you for joining.
In relation to your point about shooting modes, you may be interested to read this thread, that is currently live on the forum.
As for aperture settings - If you purchase a f3.5 - f4.5 lens what this means is that the maximum aperture you can achieve at the shorter focal lengths of the lens, is f3.5. At the longer focal lengths, the maximum aperture will adjust to f4.5. That is because of the way the lens is built.
If you want to shoot something at f2.8, then you need a lens on which the maximum aperture is f2.8. You cannot shoot at f2.8 with a lens on which the maximum aperture is f3.5.
If you have looked at equipment, you will also see that there are zoom lenses that just say, for example, f2.8, not f2.8 - f(something else). That means that you can achieve maximum aperture of f2.8 throughout the entire zoom range of the lens. And you will also notice that they are much more expensive. It is all about the technology used and the cost involved in building such equipment.
So, to summarise, the name of the lens (70-200, f3.5 - f.4.5) tells you the zoom range of the lens and the maximum apertures you will be able to achieve with it.
By the way, if you wish, you can go to Edit Profile and enter your location so that it shows up alongside all your posts, just as in my details alongside this message. Then we all know where everyone is in the world.