You can see the details of each partition/layout from the device’s DTS file: https://github.com/openwrt/openwrt/blob/main/target/linux/mediatek/dts/mt7986b-mercusys-mr90x-v1-ubi.dts#L30-L66
boot encompasses bl2 and factory partitions. It has a size of 2 MiB (1 MiB from each partition).
factory encompasses the eeprom and macaddr layouts.
You will need to have the eeprom shared by someone or you will need to dump it from a different device. I can’t help you there. The size of the eeprom layout is 4 KiB and starts at offset 0 from the factory partition.
macaddr layout is pretty obvious, since its size is 6 bytes (the exact size of a mac address). You might be able to just create a fake mac address yourself. Offset for this layout is at byte 32768.
Once you have the eeprom data, you can create the factory image using the same steps described in the wiki.