Modern devices almost always use a/b dual-firmware booting, so your flash is already split in half from the get-go. Additionally vendors tend to reserve considerable amounts of space for other partitions (beyond the basics of bootloader, bootloader environment, and wifi calibration - shared resources like download space for i18n and other things come to mind, scratch space, streamboost was popular ages ago, etc.).
Your device is partitioned for two (a/b) 50 MB ubi partitions for the actual firmware, ubiinfo will tell you about the ubi volumes within those - but that's well in line with expectations with the ~36 MB (keep in mind to add the OpenWrt image size (I just pulled 8 MB as example out of the hat, without checking) to the overlay size - and keep space for ECC in mind)). So basically all as expected and pretty similar to other devices - depending on the vendor you may see anything from 15-100 MB useable space with 256 MB NAND devices.
You have few devices with OpenWrt u-boot where you can partition ubi easily, like one partition for recovery other for normal boot.
For this - you never know how oem bootloader does with repartitioning, if you are into soldering ttl pins you can try. (ubi0 and ubi1 are adjacent, should be easy after you have that console soldered)
With SPI-NOR flash you can typically find that figure exactly, with NAND you only get an upper limit (ubi partition size, but there are multiple ubi volume within that partition). If you're lucky device page/ bootlogs or forum posts (such as this one) may provide more exact pointers.
