The chainstay bridge is the shirt horizontal connecting tube between the two chain-stays, just infront of the rear tyre.
Occasionally they have threaded horizontal holes and you can drill the fender and bolt them in from behind.
Normally you use a small metal clip over the bridge.
I prefer to use a small 3mm zip-tie. Drill a 3mm hole about 10mm from the tip of the fender, thread the zip tie through and wrap the end under the bottom of the fender. No rattle and it lasts for years.

The rack bolts can also serve as fender bolts. Put the rack next to the frame with the fender next and a washer. Use a stainless steel allen bolt that is long enough to fit right through and protrude a few threads. Grease the threads before assemly.