Do muscles grow more if you change exercises often?

Short answer

No. Whether exercises were changed every session or rotated on a plan, muscle size showed no difference from sticking with fixed exercises. But squatting alone left 2 of the 4 front-thigh muscles without growth.

Do muscles grow more if you change exercises often?
'Change often → muscles get confused → more growth': is it true?
A common belief is that muscles adapt and stop growing if you always do the same exercises, so you should switch often. We checked studies comparing changing every session, rotating on a plan, and sticking with fixed exercises.
What if you change at random every session?
Muscles did not grow more
21 people trained 4 times a week for 8 weeks. For one group an app picked exercises and reps at random each session; the other group kept them fixed. Thigh muscle thickness and the heaviest squat and bench press showed no difference, and on paper the fixed group gained slightly more.
What if you rotate in a set order?
Again, no difference
70 people trained their legs 3 times a week for 10 weeks. One group always did leg press and stiff-leg deadlift; the other did a different pair of exercises on each training day. Thigh muscle thickness rose 7.8-17.7% vs 7.5-19.3%, and the heaviest leg press 24.4-32.1% vs 29.0-30.1%, with no difference.
What about muscles already in training?
0% and 3.1% for the switched and unswitched arms
32 people did the same upper-body program for 6 weeks, then for 8 weeks one arm kept going as before while the other switched exercises or added sets. Lean mass (muscle and everything except fat) of the arm rose 3.1% in the unchanged arm and 3.0% in the arm that switched exercises, with no difference.
But 'evenly' was a different story
Squatting alone left part of the front thigh without growth
49 people trained for 12 weeks. The whole front thigh grew 11.6% both with squats only and with several rotating exercises. But with squats only, the inner and middle muscles of the 4 front-thigh muscles did not grow, while all 4 grew with rotating exercises.
Switching helped something else
Training itself became more enjoyable
In the 21-person study, the group that changed exercises at random each session showed a clear rise in enjoying training for its own sake (intrinsic motivation), while the fixed group did not. Muscle showed no difference, so the benefit of switching was fun, not muscle.
Summary: changing often does not grow more muscle
Exercises that work a muscle in different ways are enough
Changing every session or rotating on a plan gave no difference in overall muscle size from fixed exercises. But squatting alone left part of the front thigh without growth. Choosing a few different exercises for each body part and sticking with them is enough, and switching when you get bored costs nothing.