I'll share what I've figured out to this point (which I hope is approximately accurate!).
The first year of 'proper' competition in WAG is the year in which the gymnast turns 8 (this year for my daughter, so we are right at the start).
Some kids are already on the compulsory route at this point and start doing compulsory grades. There's nobody at our gym doing compulsory so that's about as much as I know there.
For everyone else, there's club grade 6 in the year they turn 8, club grade 5 in the year they turn 9, and then it splits into regional and national routes from grade 4 upwards. National is harder. Grades happen in spring - my daughter is doing her club grade 6 in April. Currently trains 6hrs a week on two days, about to move to 8hrs over 3 days.
She's got routines across all 4 pieces plus a range and conditioning routine to complete at grades. Her floor routine is her first time doing one to music with choreography like 'the big girls' - skills include splits, cartwheel x2 into backwards roll to pike, split jump, handstand bridge, handstand forward roll...
Bars is reverse grip pull up, letting go and changing grip, leg lifts, chin up pullover, circling over, arch and dish holds, fish swings.
I haven't seen her vault for ages but think it's just a stretch jump then handstand flatback. On beam she's got turns, stretch jump, caterpillar walk and she has to run and jump off.
There's also levels which I think mostly happen later in the year - there's numbers and metal names and I don't understand yet haha!
In training, my lo is just perfecting cartwheels on beam, front handspring , flicks, starting to do some work on metal bar and working on vaulting straight into handstand flatback without the stretch jump.
There's a big layer of advanced recreational in between our rec programme and the WAG squad - they do their own competitions too.
Things seem considerably less expensive than the numbers I hear from the US here (I'll be paying approx £100 a month when she goes to 3 days a week). Competition entry is about £30 typically. Haven't paid more than about a fiver to watch yet.
That is pretty much the sum of my knowledge, 2 years in!