Hi santhoshkumar siripuram,
I don't know why you say the KEA8 PTB0 and PTB1 are assigned for UART as ALT4, from the KEA8 reference manual:
https://www.nxp.com/docs/en/reference-manual/KEA8RM.pdf
page 134,

You can find the PTB1 and PTB0 ALT2 is the UART0 pin.
You don't need to configure the ALT, if you enable the UART, but didn't enable the higher priority module in this pin(AL3,ALT4,ALT6 module), the PTB1 and PTB0 will default in the UART function after you enable the UART0 module.
About the UART0_TX and UART0_RX detail, it is configured in register SIM_PINSEL[UART0PS],

in default, UART0 pin are mapped on PTB0 an PTB1.
Wish it helps you!
If you still have question about it, please kindly let me know!
Have a great day,
Kerry
-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------