The larger outfits - NYMR, SVR, WSR, Ffestiniog - do employ a considerable number of people. You cannot operate a daily service for 8 -9 months of the year without them. The majority of these will be employed as skilled maintenance staff or seasonal catering/ retail, etc. For the latter think minimum wage. For the former, less than similarly skilled people could expect to earn in the wider market.
There is some employment of operating staff, but not much. These tend to be multi- skilled guard/ signalman, and are likely to have learned their jobs as volunteers. Again, also likely to be seasonal. Footplate staff virtually all volunteers. Any paid driving/ firing likely to be done by multi - skilled maintenance staff. Some lines are seriously oversubscribed with footplate volunteers, and it can take many years to progress.
Skilled people work for heritage railways because they want to, not for the money. If cash is your motivation, you're not the right person for the sector.
To confirm the answer to your other question, heritage railways operate under LRO's, which means 25mph for passenger trains. Aside from the use of a couple of lines as test tracks, the only exception is the NYMR Whitby/ Battersby running. However, that is over NR infrastructure, and only involves their more experienced people, several of who hold full mainline competence anyway.
Reading between the lines of your original question, heritage lines are not directly a back door onto the mainline, but involvement with the more professional organisations would show any employer that you are someone who's willing to get stuck in to a job. They can also be a way for the young to demonstrate that they can handle responsibility in the workplace, or to gain skills.
Some lines do offer apprenticeships for steam fitters. Again, the people taken on are likely to be "known".
