If the phone does not charge, the schematic helps trace the path from the USB-C port to the charging IC and finally to the battery. The OPPO A15 supports 5V/2A charging (10W). With the schematic, you can trace the VBUS line to identify if the over-voltage protection diode has shorted, which is a common issue on MediaTek devices.
Given the risks and scarcity, here are practical approaches for board-level repair of an Oppo A15 without an official “UPD” schematic: oppo a15 schematic diagram pdf upd