There's an idea that people can only understand those who are like them. This is literally junk science. One of the most crucial things which makes humans sapient is the ability to learn from others, from observation, from speech, without having to experience it ourselves as animals do. We have a whole brain system, mirror neurons, which enable us to do this. Those create a sense of empathy, of sympathy, but it goes beyond that -- it's a virtual learning module. It's how we learn from things that would be dangerous or otherwise undesirable to experience personally. But there's more.
Just because everyone doesn't have the same experience doesn't mean you can't learn the same things or understand the same things. For example, I don't use a wheelchair but I've known a variety of people who do. It is not necessary for me to be sitting in the electrical wheelchair when it hangfires on a 1/4" threshold. I can gain exactly the same awareness from watching this happen, which is to say, that 1/4" is a signifcant barrier that requires problem-solving to get past. I can then account for this information in my behavior, such as looking for an easier route or asking if someone wants a push or if they'd rather sit there and tinker with the controls.
Similarly, I look white. I'm not actually all white by ancestry, and certainly not by culture, but I pass well -- until I open my mouth. So that affects the kinds of experiences I have. Clerks don't usually follow me around suspecting me of criminal intent. But I've been standing next to black friends when that has happened to them, so I know how it happens, how it causes a delay -- and that it's a bit safer for me to be the one who says, "Well, you just blew $$$ in sales." It's a slightly different, but tangent and relevant, experience of the same issue. I can incorporate that information into my behavior by not shopping at that store anymore.
Understanding isn't the same as event. It's the interpretation we make of things that happen, whether to us or to other people or most often a combination of the two. What we understand grows out of what we experience and then what we do with that information.
The reason most white people don't understand people of color is primarily lack of exposure. White folks who grow up in a throroughly mixed community do understand multicultural issues, even though their experience of those issues is not identical to their friends/relatives of color. So too, people with a disabled relative or friend may have an inside view of those issues. There are things you will simply never see unless you, or someone very close to you, has that particular trait. But not having the trait doesn't have to shut you out. It just means you're going along for the ride on someone else's experience. You can't have their experience, but you can have your experience of events brought about by the issue that correlates to their trait. This is how humans learn to understand, value, respect, and support people who are not like us. And it's a two-way street: it requires that friends and relatives be willing to share these kinds of experiences with each other by spending time together. It means making the effort to ask and answer questions that may sometimes be difficult, uncomfortable, complicated, confusing.
But if you do the work, some awesome things happen. You grow as a person. You understand the world better through the parallax of divergent perspectives, just as binocular vision is superior to monocular vision. You discover new ways of making the world a better place, and new skills to pursue that with.
This is how allies are made and coalitions are built. It's about finding common ground in the human experience of diversity. If there's a wall in the way, it's not because of what people are. It's because of what people are doing. You can't change what you are, but you can change what you do.