They’ll be using Ferrari engines for 26-27:

GM’s decision to throw Cadillac’s hat into the ring came with the caveat that its powertrain wouldn’t be ready until 2028—two years after it actually wants to enter the sport. That means for 2026 and 2027, Cadillac F1 will use customer engines from another manufacturer, in this case Ferrari. From 2028, we can expect a GM-designed V6 hybrid under Cadillac F1’s engine covers.

https://arstechnica.com/cars/2024/11/cadillac-f1-will-be-able-to-race-from-2026-despite-previous-snub/