boot – Install grub on a multi-partition multiple partition USB flash drive

I have the following USB flash drive partitions:

/ dev / sdb1 * 204800 8593407 8388608 4G b W95 FAT32
/ dev / sdb2 8593408 11460607 2867200 1.4G b W95 FAT32
/ dev / sdb3 11460608 24043519 12582912 6G 7 HPFS / NTFS / exFAT
/ dev / sdb4 24043520 120127487 96083968 45.8G 7 HPFS / NTFS

sdb1 = Lubuntux_64 with persistence.
sdb2 = androidx_86.
sdb3 = Windows10_64.
sdb4 = General storage.
With 100 MB unassigned before sdb1.

Why is it like this? I tried several multi-boot softwares, but none of them worked as expected. This configuration works, but I have to manually set the start indicator in each partition I need to start.

The question is: How can I install grub in sdb (which detects all operating systems) for a menu that allows me to choose which operating system to run? I am not afraid to write things manually, but I do not know what to write in grub.cfg to indicate the necessary partitions.