If you google various combinations of "vegetable companion chart
intercropping" you find many copies of a chart that comes up at the top
of the search anyway. There are definitely interactions between
veggies. This summer I could see that pole beans in one foot of
compost, near garlic, grew and produced only a half than those four
feet away, which had only a few inches of compost and acorn squash as
companion. But if you look at different tables, you find that cabbage
and tomatoes are listed as enemies in one, and companions in another. I
was also very frustrated to see very little .edu sites discussing the
matter. What is your experience? Are cabbage and tomatoes friends or
foes? What about the rest?

I don't know about cabbage and tomatoes, but I once read that asparagus
and basil were good companions, so I tried it. It didn't seem to
affect the asparagus much one way or the other, but the basil grew just
beautifully! I had a friend try it with the same results.

That too. Companionship can be one-way. Here are a few things I see:

- bokchoi traps a lot of bugs. Good for nearby brassicas, but not so
good for the bokchoi itself.
- parsnip goes well under tomatoes, despite lack of sunlight. Tomatoes
unaffected.
- collard flourishes under peas (this was an obvious one), but the peas
don't gain a thing.
- squash not affected by beans, despite claims of the contrary.
- lettuce goes well with garlic.

One other thing I have noticed is that brassicas by themselves
inevitably get skeletonized and die (lots of cabbage moths around
here). But brassicas all together survive. And the explnation is
obvious: robins visit a large cabbage patch, but will not check
individual sparse plants. So many cabbages are good at keeping each
other company.