I don't have any recent experience (my DD spent her gap year working at a US summer camp), but back when I had my gap year I worked for part of it and then spent 11 weeks travelling on one of those tours to Kathmandu.
This was back in the late 1980s though and I don't think my experience is very relevant to what modern day conditions will be like.
You will need to check out if the particular tour is a more luxurious one where they will be staying in hotels, guest houses etc or if it's the more traditional type that involves camping in two person tents and helping with the cooking and washing up.
If it's the type where they are staying in local accommodation then I guess it wouldn't really be any different from any other sort of guided tour for a general group except maybe they prioritise activities that would appeal to younger people than to pensioners on a coach trip.
In contrast, if they're travelling in the back of an expedition truck, camping in two-person tents and having to help with the cooking etc then your DCs will need to think about whether that is something they actually want to put up with in a holiday.
Some tours may be somewhere between those two extremes.
The description for each tour should tell you exactly what the accommodation is and what the transport is etc.
As I said, when I was 19, back in the late 1980s, I worked for part of my gap year and then signed up for an 11 week overland trip to Khatmandu, the capital of Nepal.
We travelled down through Europe to Turkey, then through Iran to Pakistan, India and Nepal. We travelled in a bright orange Bedford truck with a trailer that contained all the equipment. The whole time was spent camping and we would stop off and take turns to being involved with buying fresh food and supplies from local markets or doing the cooking etc.
That's clearly one end of the spectrum and I'm sure that there is a very different, much more luxurious end of the market.
I didn't go on a specific 18-30 thing but most people were in their 20s or 30s. At the age of 19 I was the youngest by at least two or three years. It was certainly an experience and I think I grew up a lot over the course of those 11 weeks.
So, all I can say is to read carefully the description of individual tours, the accommodation and travel arrangements, which will give you an idea of what sort of tour it is, a fancy one staying in hotels or a back to basics one where you're sleeping in tents.