In most cassette cogs ( 8/9 speed) the largest seems too large ( mostly in Shimano)---does it really function? I mean , it skips A LOT---?

You shift cable as stretched and wont shift to the largest cog. Very common on bikes especially when new.

There is a cable barrel adjuster right where the cable enters the rear derailleur. You want to turn it out 1/2 turn at a time and try it. It may take several tries.

DO NOT touch any screws.

Mine work just fine.

Odds are, something on your bike isn't adjusted right. I'd start with the derailer, pretty much as OH suggests.

Or do a net search for "rear derailer adjustment". Plenty of good material available.

Another possibility, but lower, is mismatched wear. Sprockets and chain wear together. If there's one gear/sprocket you "never" use, the chain might not longer fit properly on that one.

If you never feel the need to use ALL your sprockets, consider getting a cassette with a tighter spacing for best use of your drivetrain.

Derailleur cable tension adjusted wrong - most likely cause. Turn the barrel nut adjuster counter-clockwise 1 full turn. If that doesn't get it done - continue in 1/4 to 1/2 turn increments.

none of them should be ''skipping'' ever

i use mine all the time

all of them

wle