So my firm has a private investigator on staff. Most of what she does is background checks and reviewing discovery, sometimes tracking down and interviewing witnesses. She's also a paralegal, so she does paralegal stuff too. Its pretty common for medium and large firms in my area to have such a person on staff.

However its not necessary to have a PI to access the type of info you're talking about. As part of litigation attorneys will engage in discovery, meaning they will make requests that the other side produce documents and records and submit to depositions and interrogatories (witness questioning). This helps each side build their case for trial (or confirm their case's weaknesses as it were). Documents and records can also be requested from third parties who may be in custody of them, and third-party witnesses can be independently deposed. For example, subpoenaing Facebook for someone's messages or requesting investigation records from a relevant law enforcement agency. Discovery tends to be expensive and time-consuming, so it can be, and oftentimes is, used as a motivation to get the parties to settlement.