The RNG is no longer a shared variable in the rng_fn function. It’s either a numpy RandomState or Generator (usually the latter as the RandomState is being gradually abandoned by NumPy). You can take draws from these objects by calling rng.foo like rng.beta(alpha, beta, size=size). Most distributions are available in both Generators and RandomStates, but you can check the numpy API to confirm (some have different names, like integers and randint).
If you want to use scipy methods you can pass the rng as in scipy.stats.beta(alpha, beta).rvs(size, random_state=rng)
Finally, you can also use other RandomVariables rng_fn class method.