OP-TEE is automatically started at boot time by TF-A, and runs as the Trusted Execution Environment. See this page on the ST32MP1 Wiki for an overview of OP-TEE on STM32MP1.
From Linux, OP-TEE can be tested using a few example OP-TEE applications. available only in the demo configuration.
# optee_example_hello_world
This is a very simple Trusted Application to answer a hello command and increment an integer value.
# optee_example_random
Generates a random UUID using capabilities of TEE API.
# optee_example_aes
Runs an AES encryption and decryption from a TA using the GlobalPlatform TEE Internal Core API. Non secure test application provides the key, initial vector and ciphered data.