If you check the electrical diagram, on page 3, there is the PTB0 pin connector to the UART0_RX_TGTMCU, following the connection to page 4, it says that it connects to the UART0_RX_TGTMCU_BUF and this to the UART0_TX_TGTMCU_R of the OPENSDA MK20DX128VFM5. You will not find it on the arduino connector.
Hi @Andrei13 you can find it on the next page
https://www.nxp.com/downloads/en/schematics/FRDM-KE16ZSCH.zip
Have a good day