Hello Alex,
When you send in your phone to OnePlus they always wipe the data. It’s in their ToS, when I sent in my OP6 last year for battery replacement they warned me about this beforehand.
''“Your data will be erased during the repair process. Please back up all personal data on your device.“''
Source: https://www.oneplus.com/ie/support/repair?from=foot — Under the '''Request Repair''' button
The second issue is that the data is most likely encrypted on the phone. For example with iPhones you need the cpu, nand, eprom etc just to access data. Furthermore, the memory chip is soldered onto the motherboard, no easy way to access it.
As to your broken phone, your best bet is contact OP again and ask they if they could fix it.