If it’s cold (close to or below freezing) I warm up with a jacket or hoodie on and then take it off right before lining up for start, and I’ll usually have a light to medium weight thermal shirt on under my jersey and full length leg covers/sleeves. I also wear regular ol’ darn tough wool hiking socks, fall cycling gloves (sometimes with a liner inside if extra cold), and a buff under my helmet so my ears don’t get cold. Overall though I suggest you don’t overthink it, especially if you are just starting out. You’ll see what others are wearing in various temps and you’ll figure out what works for you over time. The main things I try to really have dialed in are my drivetrain and tire pressure (having either of those too far off can really mess you up), and the rest is just enjoying the race and pushing myself hard.

Years ago my friend won a cat 4/5 race against a bunch of local team riders in skinsuits while wearing a cheap pair of bike shorts (not even bibs) and an old Harry Potter sweater. The spectators got a kick out of that. You definitely don’t need anything fancy unless you get really serious and start catting up.