I have a multithreaded Spring Boot microservice running in a Kubernetes pod with a CPU limit of 1 (1000m). Does this mean only one CPU core is used to run all my threads one by one, or can multiple cores run my threads concurrently as long as the total CPU usage doesn't go over the 1 CPU limit?
multiple cores [can] run my threads concurrently as long as the total CPU usage doesn't go over the 1 CPU limit
This. You can have as many threads as you'd like, and the kernel will schedule them on as many CPU cores as are available, but your process won't be allocated more than 1 CPU-second per second.
Say, for example, that your application is largely database-bound, and a thread can only practically use 25% of a core. If you had 4 threads running, together they'd still fit inside a limits: { cpu: 1 } constraint, and they would all get to run at their respective full speeds (maybe on different physical cores).