As a math amateur, I am finding the study reduced residue systems relative a primorial a very interesting way to understand the distribution of primes. For example, it is fascinating to me that it is so easy to count the number of integers $x < p\#$ where $\gcd(x(x+2),p\#)=1.$

When I do a google search on reduced residue systems, I find very little that is interesting. On this site, for example, I see only 12 results.

What is the reason that there seems to be little active research on reduced residue systems modulo a primorial? Is it that the material is so well-studied that it no longer has much appeal? Is it that other areas seem so much more promising? Is it that research in reduced residue systems relative primorial is now focused on highly technical topics?

Not knowing a better source, I have decided to crack open my English translation of Disquisitiones Arithmeticae. It is surprisingly clear and easy to follow (it probably gets more difficult later on). Assuming that research on this topic is no longer active, are there any other classic works that are recommendeded to provide an amateur with a deep insight into reduced residue systems relative a primorial?