1st of all - always disconnect battery connector from logic board(!!!) specially when you installing/replacing HDD flex cable!
So, all cables(5pcs or more) which is replaced with battery connected to motherboard and without ground wrist band(also very important!) can be totally dead, or maybe 50X50, now you need to check all this cables on 100% working MacBook to be sure which one is alive.
Be sure you have 100% working and tested USB drive, HDD cable & Hard Drive or SSD.
Insert it all to your MacBook, push Power button and then push & hold Command until you can see drive options for boot on MacBook screen. Here just choice the drive and click it to boot.
If you can choice from this screen your preferred boot drive(USB, SSD, HDD) and it do not boot from this drive but show you Folder logo, then this drive do not have bootable MacOS partition, so, you just need to format this drive, make it bootable and write OS to it.
All known problems related to your issue -
broken HDD
broken SSD
broken FlashDrive
broken USB port or bad soldering
broken HDD cable
wrong HDD/SSD/FlashDrive format or not bootable drive
broken logic-board - usually bad soldering contacts of SouthBridge, need to be reballed or replaced.
Please be sure that all your parts(drives & cables) fully functional before you try to solve this issue!
In other way you can gone in repair loop ;)
Wish you good luck!