firstly, beef tendon loses moisture after being heated by microwave oven, and becomes dry and hard.

Second, beef tendon contains collagen and other mixed substances, which will make food dry and hard when it loses water. This is why it feels so hard.

So it can basically be ruled out that restaurants add other things to food.

I hope you are satisfied. Reminder, it is best not to continue to eat leftover food. If it takes a long time, it will easily produce nitrite, which is harmful to the health. It is also the reason why overnight dishes become salty.