Trying to upgrade a 2021 giant ascend at to Shimano 105 without changing the wheels. The rear wheel only has a 9spd cassette, as I’m upgrading from 9spd Sora group. I’ve found Shimano 105 R7020 11spd levers and derailleur but want to be sure they work with the 9 spd cassette. Thanks
11-speed shifters almost certainly won't work with a 9-speed cassette. Aside from the obvious issue that you'd have two extra clicks on the shifter (which I suppose you could opt not to use, all other things being ok), the main problem is that the derailleur won't move by the correct amount with each click of the shifter.
An 11-speed shifter paired with an 11-speed derailleur, when clicked one position, pulls the chain across exactly to the next sprocket on an 11-speed cassette. But the spacing between sprockets on a 9 speed cassette is different, so with 11-speed shifter+derailleur the chain will end up not correctly lined up with the sprocket in at least some gears, which will cause skipping/rubbing.
It won't work with a 9-speed derailleur (and 11-speed shifter) either, because a click on the 11-speed shifter doesn't pull the same amount of cable as a click on a 9-speed shifter, which it would need to do (~2.78mm vs ~2.55mm by my calculation, based on the pull ratios documented https://bike.bikegremlin.com/1278/bicycle-rear-derailleur-compatibility/ and sprocket spacing from here http://obike.dk/teknisk_info).
Note that because the pull ratios are different, you also can't just swap the derailleur out (leaving the 9-speed shifters and 9-speed cassette), which otherwise would've been an easy upgrade.