I am near the end of my career and have mentored many people along the way. I have also had several mentors. Mentors can be incredibly helpful and provide a genuine advantage to their mentees. I think it is very important to understand what you are looking for in a mentor and in what aspect of your life. For simplicity, I would segregate it between a work mentor and a non-work work. The non-work mentor would help you matters such a relationships, family matters, work/life balance, etc. I have not had much success with non-work mentors. I tend to turn to friends and family instead.
Some companies will assign you a mentor, particulary if you are in a group that historically does not utilize mentors much. I have not seen this work very well. First of all, there are never enough senior people to be mentors to the vast number of mentees that participate in the programs. Each mentor has to "cover" several mentees leading to less time spent with them and less investment by the mentor.
A good mentor is someone relatively senior and definitely not your boss or your boss's boss (grandboss?). You want someone who has seen/experienced many things. Someone who has seen and understands things like office politics. You also want someone who will be honest with you. You need them to be invested in you.
These relationships can start from either side. I have seen young, talented people that look like they could use some assistance and reached out to them to offer my help. Usually they accept as I was a relatively senior person by the time I started doing this. I was looking to pass on my hard-earned experience and knowledge to someone I thought would use it and benefit from it. The best relationships I had as a mentor occurred when the mentee asked me and not as part of a program. In these cases they were motivated to learn and improve. Most importantly, they acted on what we talked about. To be clear, a mentor does not tell you what to do. They are a sounding board that has seen many more situations than you and can help you think things through. If you never act on their advice or always argue against it (Hey Boomer!), you are simply wasting their time. A good mentor always has other things to do - their time is a valuable gift.
As for what you can learn from a mentor, it depends upon the mentor and for what you are looking. I found mentees frequently did not understand how decisions were made at the senior level and/or what the corporate strategy was. They did not understand how what they did fit into the bigger picture and how important what they did was to the company. Some, usually new managers, wanted to understand how to motivate and retain their employees. Others wanted to understand KPIs that they could design a measure for their team. Others wanted to understand how they should think about promotions and differnent job opportunities (even outside the company). Each one was different.
The single best mentee I ever had was at a company where we all worked remote. We had regular Zoom discussions every couple of weeks (in office jobs, I preferred to go to lunch or get coffee with a mentee to get away from the office). He showed up prepared - an informal status on what was going on with him and his team, a list of questions/topics to discus, and feedback from implementing what we discussed in prior meetings. He was very thoughtful about what he wanted to learn - how to be a better manager, how to prepare himself to be better positioned for promotion, how to consider various career opportunities. After a year, he surprised me with a summary of what we had discussed, what he had done differently as a result of our conversations, and what was the impact. He even walked me through his metrics to describe his success. It was phenomenal. Incredibly satisfying for me as the mentor. I was definitely invested.
One final thought in the long post - mentors who are invested in you will work to advance your career when you are not around. They will recommend you for positions of which you are not aware (this is how many positions are filled). They will refer you to friends and associates at other companies where they think there is a great opportunity. They will advocate for you. This is an incredibly valuable aspect of the mentor/mentee relationship.
For success in mentorships you need to be invested, you need to get your mentor invested, you need to be comfortable having honest discussions with them and believe that nothing will be repeated to others, and you need to act on what you discuss.