who knows where I can download its SOC schematics?
You can find the **board (**not SOC/SOM) schematics at https://www.96boards.org/documentation/consumer/dragonboard/dragonboard845c/hardware-docs/
where can I find its SOM schematics? can you share UFS part schematics?
the SOM on the DB845c (or RB3 platform) is a standard product from Thundercom, e.g. the TurboX D845 SOM, I recommend you ask Thundercomm directly if they can provide the schematics.