Can you gain muscle doing only isometric exercises?
Oranchuk et al. (2018) performed a meta-analysis of twenty-six studies and found that "substantial improvements in muscular hypertrophy and maximal force production were reported regardless of training intensity".
maximum voluntary isometric contraction (MVIC)
Unfortunately "most studies (16/26) recruited untrained participants, while the remainder (11/26) utilized "active" or "recreationally trained" participants", so the evidence isn't strong whether isometric training can result in hypertrophy in well-trained individuals.