Lots of symbolism but what does it mean?

Per your general question, regarding symbolism:

There was heavy emphasis symbolism in The Neon Demon, which I believe is one of the most attractive aspects of this film – and also a reason I and others had to see it more than once.
The moon, eyes, predatory cats, triangles, binding, strobe lighting, the number three and other symbols could be considered code for characters and/or were often foreshadowing of future events in the film. When combined with colors (also "symbols" of sorts) the story starts to unfold in layers.

As for your specific questions:
a) Ruby was the head of a three pronged (triangular) coven. She is an Elizabeth Bathory-type character. There are many references to witchcraft, ranging from this example to the significance of the moon (another topic that has several layers).

b) Although I can't comment on the specific tattoos, there were circles, symbolizing the envelopment of Jesse via the contrast of black and pink. This theme played out twice during the Neon Demon:
**Pink had earlier been used in dirtier (or a more salmony) pink on the motel walls – noting the clashing of Jesse’s naivete and the grittiness of her environment. When she witnessed the horrific occurrence in the next room, the pink area became encircled by a much darker force... BLACK, and shrunk down as her innocence was disappearing.
**Note that black was used again encircling Ruby’s (pink) nipples in the bath and the garden scene (evil surrounding youth/beauty/virginity)

On the topic of color:

1. The demonic RED is appropriately the dominant color. I think that Ruby (noting the name) had some article of red clothing in every scene until she was literally bathing in it; this was then followed by black, then pink/green. Flashing red lights during the stage demonstration as ‘Demon Dance’ was playing may have represented a warning, and Jesse saw herself in the red triangles during the pivotal and hypnotic runway scene.
2. BLUE represents narcissism from Narcissus, Caravaggio's classic Greek mythology painting. We see significant shifting between blue and red in phases many times throughout. I think the most poignant (and beautiful) use of blue was in the final scene. Sarah, previously seen as "beige" and rejected in the test scene (see the below beige reference), had just “eaten the competition.” She tried to hide her confidence as Jack asked for her to replace the other model in the poolside (also blue) shoot. It was at this time her eyes were incredibly blue, accentuated by a blue curtain in the background. As Gigi became sick, she entered a blue-tinted area, vomited the eyeball (the iris of which was blue), which was consumed by Sarah.
3. WHITE was the dominant color in the early and brief scene in which Jesse is walking in the corridor of Roberta Hoffman’s modeling agency. She was still pure at that time, only to subsequently relinquish to conformity (albeit temporarily)…..
4. BEIGE: the non-descript color of ‘undifferentiation’ was used as the mannequin-like models awaited their opportunity to separate themselves from the others in front of the judges.
5. GOLD, associated in both Eastern and Western culture with perfection, is the paint color that Jack chose after gazing at Jesse. It was at this point that Jesse was starting to buy into the hype about her beauty.
6. GREEN is a color of serenity, used heavily for the first time with Ruby in the garden, likely representing the first time that she is at peace. [Note also that Ruby's prone position on the grave was consistent with Jesse's life force (4th Chakra in the Hindu religion, with dominant colors of green and the below pink)]. There previously were hints of green in the pink flowers given to Jesse by Dean - noting that she had discarded them earlier in casual act of hubris.

It's about satanic pedophilia, which is actually happening in Hollywood in real life.