Who picks judges and how are they paid?

depends on the State, but for the most part the commission assigns them. The commissions tend to cooperate somewhat with the sanctioning bodies when it comes to title fights, and select officials that are approved by the sanctioning body. But that is a courtesy, and does not always happen. If a promoter or manager has a problem with any official selected, you can object to that official, if you can provide cause or demonstrate previous bias, etc. Sometimes the commission will remove that official, sometimes not. As for fees, the commission sets the officials fees and travel expense. Typically the promoter does not pay the officials directly. The commission gives the promoter an invoice, the promoter pays it, and the commission will then distribute the pay to the officials. Some states may vary slightly on the process. In Florida for example the promoter would write out the checks directly to the official, but give the checks to the commission who would review the checks for any improprieties and then give them to the officials

They’re selected by the commission’s but essentially have to be approved by the promoters, it’s very common for the promoter to pay their expenses like hotel’s, travel etc. That’s often cited as a way of influencing judges.