If ST Micro uses custom HCI commands to create services then it means they are running both Host and Controller.
For a platform-neutral Host that can work with any standard HCI device (including Zephyr), check out nimBLE:
From: users@... <users@...> On Behalf Of valens.dsilva via lists.zephyrproject.org
Sent: 29 July 2022 01:10
Subject: Re: [Zephyr-users] Porting Host-only build with HCI SPI to another RTOS #stm32 #nrf52840 #hci #spi
it seems ST Micro (for their BlueNRG chipsets) uses custom HCI Vendor commands (ogf 0x3F) to create services so they are running a hybrid Host and Controller in their BlueNRG chipsets. Is there an open source BLE host that works with Zepher's