I believe that I am simply missing some part of the documentation.
Spring @Profile says:

… If no profile is active using one of those options, a default profile
is enabled as a fallback. The name of the default profile is “default”

The simple example:

@Component
public class Config {

    @Bean
    void a() {
        System.out.println("a - empty");
    }

    @Bean
    @Profile("default")
    void b() {
        System.out.println("b - default");
    }

    @Bean
    @Profile("dev")
    void c() {
        System.out.println("c - dev");
    }
}

Run with empty profile:

No active profile set, falling back to 1 default profile: "default"
a - empty
b - default

Run with using a default profile:

The following 1 profile is active: "default"
a - empty
b - default

Run with dev profile:

The following 1 profile is active: "dev"
a - empty
c - dev

Could you explain to me why does it pick the “b” in the second case and does not in the 3rd?