Others have given excellent advices. I'll approach it from management point of view:

• If there's management oversight, such as tech lead/engineering manager, talk to them. Don't make any accusation. Approach it from the direction of you feeling uncomfortable with how the team is working. They will know how to solve the issue. However, any tech lead/engineering manager should have already dectected the problem and at a minimum acknowledge the issue.

• If there's no tech management oversight, I'd suggest you approach the senior engineer directly. I'd want to emphasize here that it has to be tech management. Non tech management won't understand the problem and they won't be able to solve the problem. Sometimes the senior engineer maybe under pressure to deliver and there's nobody to split the tasks to other team members. I did this a few times in my career before I developed my skill to lead a team.

• If it's neither because the senior is under pressure to deliver, nor there's management oversight, your next best bet is to seek consultantion with another senior, either in your team or another team. They maybe able help to talk to the senior.

• Your last resort would be non tech management, or saying it another way: express that you're not happy with your job. This won't be much help unless others in your team doing so as well.

If all these fail, consider finding another offer. There's no oversight, there's no willing to inprove from the senior and there's no chance to improve the situation from other seniors, you won't learn much there.