Building muscle is simple. Lift heavy things, rest, make sure you eat enough food, sleep, repeat. For a beginner, progress is linear and relatively sudden. You get quick feedback: your muscles get more defined, you look a little leaner, you can lift a little more each session, friends and co-workers notice and comment on the changes. New striations pop up, clothes fit differently, you feel more capable dealing with the physical world. You’re hungrier and heavier, yet still manage to drop belt sizes. All is well.
Muscle isn’t the only thing you’re impacting when you lift heavy things, though. You’re also imposing stress on your tendons and demanding an adaptive response. You’re training your tendons, too.