How do "investigators" of American law firms investigate cases?

Based on the judicial environment in the United States, public prosecution lawyers (that is, prosecutors) can ask and guide the police to investigate criminal cases and collect evidence; Defense lawyers can hire "investigators" to investigate the facts of the case and collect evidence. With the consent of the police, "investigators" can enter the scene to investigate and collect evidence. According to the law, under normal circumstances, the police are not allowed to set up obstacles. "Investigators" generally have professional knowledge of investigation, judicial background and are familiar with the investigation process. They will use any effective means to investigate the case within the scope permitted by law. If you need to know in detail, it is recommended to watch relevant American dramas. Although it is exaggerated, the systematic interpretation is still within a reasonable range.