Yes, online races do use your garage cars for the Gr. 3 & Gr. 4 races. The N300 are one make races and you are provided the car. This is because in the first week, the N300 class was at Brands Hatch but people were de-tuning the BMW M4 to N300 class and it would **** all over the N300's. So they changed it to a one make class/car combo and can not tune the car.
They have now changed the daily races to being actually daily races. So far since release we've had the daily races change 3 times. Now they are changing daily, which is fine and as the name suggests, daily races, but they need to be up at least 3/4 days imo. You can't learn new car/track combo everyday and expect everyone to know the braking points etc, just creates chaos. They need a lot more variety too instead of 3 races, maybe 6 races, different car/track combos and change them every 3/4 days.
I've almost done the campaign mode, just got the last 4 missions left on stage 8 to do on the Mission Challenges.
I've been loving the online races I had been doing though when the Gr.4 was at Kyoto Driving Park. Had so many good races, a few poles and wins too which is good.
I've done 66 races, won 9, 5 poles and 13 fastest laps in online races. Think about 55 of them races was at Kyoto in the Gr.4
This was my closest win.