Companion plants are a means for nurseries to sell more plants. It is a marketing ploy which caught on with consumer gardening publications. Of greater interest are indicator plants which become stressed prior to the majority of your garden or orchard.
Nitrogen fixing plants do not put nitrogen in soils, but rather enlist specific soil microbes to “fix” (preprocess) nitrogen sources on their behalf. Here’s a post on the subject from awhile back: Diversity among Nitrogen fixing plants
“Cover crops” is a valid topic from which marketing of companion plants evolved. The purpose of a cover crop is to suppress unwanted growth (i.e. weeds) between rows of orchard trees or rows of annual produce. Typically the “cover crop” is tilled under after harvest. The cover crop is very specific to the target crop you are growing, plus your local soil and climate. Sometimes it has benefit beyond weed suppression but usually not.