This information is easily google-able. All of the crew are salaried and this is generally tax-free as they're 'off shore' and in different tax jurisdictions.
The charters that are filmed for Below Deck are at a 50% discounted rate, so a £100k 3-day charter would be £50k instead if filmed. This is why you sometimes get 'estate agents' and random smaller business owners appearing on BD and milking the booze/food/experience, because they can only afford it with the sizeable discount. The costs of running these yachts is astronomical, fuel, tech, crew, docking fees and repairs are all £1000s per week.
The tips are part of the contract for BD but the guests can decide what they feel is appropriate.
The crew salaries can be seen here as a guide: www.crewfinders.com/yachtcrewsalaries.shtml
All of the crew are licensed to work on the yachts although some are very green. Obviously the production company choose the more entertaining personalities to keep the show interesting, which is why the guests don't always receive the experience they would on a non-TV show boat.
Friend is in the yacht sales business and I know lots of crew members. The crew on BD also make money from insta posts etc & interviews.