How was she able to be the only pop girl that debuted during the post 2003 new Urban artists like Ciara, Cassie, Chris Brown, Amerie, Teairra Marie e.t.c who still is able to sell millions and have crossover smashes?

Umbrella.

Good management, looks and controversy.

Catchy singles, sexy image, nice budget, good promo spots

The support of her management, the support of jay-z, being talented I guess xD ,I like Ciara

What controversy? CB incident?
If so why isn't Chris selling half as well as he used to too?

She found her lane

Her first two eras she was a sort of Beyonce second rate. Beyonce was her idol at that time so it was understandable. With GGGB she made her own lane, she isn't only urban she massive crossover success garnered WW appeal and 10 years later she's still here. Sometimes I shade Rihanna by congratulating her team but truthfully it takes more than a team to make someone a star. Rihanna is a smart woman.

Good management?!
Y'all realize Rih was about to be dropped twice?

She turned to Pop, her management and label were on point since GGGB and she has an ear for hits.
A good product, indeed.

Jay Z, Ty Ty, and Jay Brown!

Loyal fans

During the mixtape eras??

Yeah during MoTS and AGLM, she didn't get support till umbrella, which never even got great promo apart from an Mtv performance.
Plus any person whose followed Rih's albums knows GGGB and LOUD are tge only eras that weren't messy, the rest like TTT and unapologetic never got any promo minus the first two singles
Therefore this "management" thing doesn't make lots of sense.

Bajan voodoo

I'm a bit confused as to why Rihanna is grouped together with actual urban artists like Ciara, Cassie, Teairra and Chris Brown. I mean, I know she is black, she's performed with Amerie and Teairra Marie before and dated Chris Brown, but the majority of her career has been in making POP music and only recently has she started doing urban which basically answers the question asked.

She went pop, sexed up her image, became unique, found her own lane, and stayed in it. She hasn't really done anything to controversial to end her career, if anything the controversy she's endured has helped her career.

Knee pads.