You need to read the PPS section, in particular pages 210 onwards. Inputs and outputs are mapped differntly which can be confusing. So, the UART receive pins are mapped using the registers on page 211 - RPINR18 and RPINR19. They can be mapped to any of the RPxx pins.
Output mapping is page 215 onwards.
I seem to think there are some RPxx pins that can only be inputs, not outputs, so the RX1 could be mapped to RPIxx but not TX1.
I also seem to remember I2C pins are fixed.
Keith