The market is very competitive- capacity can get tight, which makes room for the “little guy” to grab a load. Even at the biggest companies with millions of lines of software, tons of logistics is still driven by email.
Job boards are big business. They act like a 4th party logistics company, where they aggregate the available loads from multiple 3rd party logistics companies. But there’s plenty of these aggregation site that, as mentioned operate a lot more like Craigslist.
More sophisticated systems are like the stock market, automated systems booking loads within one second after becoming available.