Since lens with high focal distance are generally not that fast and they need high shutter to freeze moving animals, how do they get light without addind too much gain an grain ?
what do you mean? What’s so strange about say 1/1000s f/4 in daylight?
There’s no issue what so ever.
Pro wildlife photographers will spend tens of thousands of dollars to get a faster lens.
Flash. No kidding. Just a smidge.
Practice and money.
Better body with full frame has less noise, but costs
Better lenses with lower Fstops, but costs
Learning the lighting situations.
Editing in post.
They fill their memory cards with thousands of pics and cull down to the best ~10%
People don’t see the other 900 throwaways, they just see the good ones. So they think wildlife photographers go out there and click a few perfect pics and then go home.