Here's a breakdown of how Mona could have gathered information:
1. Early "A" Activities: Mona was initially believed to be "A," and she was shown to have access to information and technology that allowed her to harass the girls.
2. Association with "A": Mona was later revealed to have been working with the real "A" (Alison DiLaurentis) for a period of time, giving her access to Alison's knowledge and resources.
3. Personal Connections: Mona was a close friend of the girls and had her own sources of information. She was also aware of their secrets and vulnerabilities, possibly through eavesdropping or blackmailing.
4. Observational Skills: Mona was portrayed as an observant and manipulative character. She likely used her knowledge of people and situations to her advantage, piecing together information over time.
5. Blackmail and Manipulation: Mona was known to blackmail and manipulate people to gain information. She likely used her knowledge of their secrets to force them into doing her bidding.
6. Technology and Hacking: Mona was shown to be tech-savvy and could have used various methods to hack into computers, phones, and social media accounts to gather information.
7. Multiple "A's": Throughout the show, it's revealed there were multiple people who played "A," including "A Team" members. This suggests Mona may have been privy to information from various sources.
8. "A" Legacy: Mona's time as "A" likely left her with valuable information and resources that continued to be helpful in later seasons.
9. Theories: Some fans theorize that Mona may have been a pawn in a larger game, manipulated by someone else to do their bidding.
In conclusion, Mona's knowledge of "A" and the secrets of Rosewood is a mix of personal connections, manipulation, observation, technology, and potential association with the original "A." The show doesn't fully reveal the extent of her knowledge, leaving it open to interpretation.