They train exactly the same as other tendons. You progressively overload, they adapt – it’s just the time scale is much longer than muscular adaptation. Also, many muscles around the upper body attach to deep fascia and even to other muscles rather than just to bone, so their way of stretching (per pistols example) might not work the way you think.
But as Robby said, I wouldn’t get too hung up on this aspect unless it’s something you’re studying. Nice to know but just having faith that your body knows what to do, how to do it and will respond to training is enough 😉