You can decode a raw binary STM trace file and save the result as a text file.
The stm subcommand writes decoded data to standard output; redirect that stream to capture it in a file.
For default output behavior during live capture, see trace stm.
The following command decodes stm_trace.bin and writes the output to decoded_trace.txt:
nrfutil trace stm --input-file stm_trace.bin > decoded_trace.txt