Tune customs Tunings

Hi together,
Forgive me I am German so the terminology might not be precise.
My question is, as I am not an expert yet, I have trouble tuning my cars so they are real competitive.
I found the option to utilize some pre-sets from other tuners so for the BMW 5 series which I love to look at I applied the best A800 pre set (best values in the star (accel…, speed, breaks)
But it won’t come close to my Mercedes C- (c204) Coupé (not the new one which turned out to be shi… although I love its look)
Can I somehow choose the pre set and take it from there and improve the BMW? Because when I try it always says I can’t change the pre set. However I also possess all the car parts whenever I have purchased the pre-set right?

Also I thought the other cars always depend on my own one so shouldn’t I be able to win races with any car as long as I make the best of it?

Thanks in advance for taking on my noobish questions:)