74HC595 to 4 Digit 7 Segment using SevSegShift Library
Finally able to fix it with the help of @PeterPaulKiefer, I was able to find the problem. The problem is in the code itself (schematic is alright. No changes there) and this is the updated code.
#include &lt;SevSegShift.h&gt;

#define SHIFT_PIN_SHCP 6
#define SHIFT_PIN_STCP 5
#define SHIFT_PIN_DS   4

SevSegShift sevseg(SHIFT_PIN_DS, SHIFT_PIN_SHCP, SHIFT_PIN_STCP, 1, true);

void setup() {
  byte numDigits = 4;
  byte digitPins[] = {12, 11, 10, 9}; // These are the PINS of the ** Arduino **
  byte segmentPins[] = {0, 1, 2, 3, 4, 5, 6, 7}; // these are the PINs of the ** Shift register **
  bool resistorsOnSegments = false; // 'false' means resistors are on digit pins
  byte hardwareConfig = COMMON_ANODE; // See README.md for options
  bool updateWithDelays = false; // Default 'false' is Recommended
  bool leadingZeros = false; // Use 'true' if you'd like to keep the leading zeros
  bool disableDecPoint = false; // Use 'true' if your decimal point doesn't exist or isn't connected
  
  sevseg.begin(hardwareConfig, numDigits, digitPins, segmentPins, resistorsOnSegments, updateWithDelays, leadingZeros, disableDecPoint);
  sevseg.setBrightness(100);
}

void loop() {
  sevseg.setNumber(500);
  sevseg.refreshDisplay(); // Must run repeatedly; don't use blocking code (ex: delay()) in the loop() function or this won't work right
}

What changed:

Turns out my 7-segment is COMMON_ANODE so I changed my hardwareConfig
Set brightness to 100
(At this point, it shows the number in reverse but with a decimal point in each digit)
Set disableDecPoint to false
(At this point, the display is showing but in reverse but no more decimal point)

To fix this, I set the digitPins[] in reverse and it now shows the proper format


Thank you so much Peter for helping me figure out what's wrong!
Also the LED Bar I use is 4-Digit 0.36&quot; LED, 7-segments display, common-anode. (The link is to the shop I got it from)