Key storage in non-TFM mcu

In my previous project on a MCU with TFM, I used the PSA based APIs to store and retrieve keys, e.g. psa_import_key. I think the medtls links with the TFM code and TFM handles the key storage, etc. in the background.

But for non-TFM mcu, what APIs I can use? And where the keys will be stored eventually?