BigInteger.js | An arbitrary length integer library for Javascript | Math library

I'm using this BigInteger.js for some calculations:

I saw Explain BigInt Like I'm Five, but I already understand what a BigInt is. I want to know how to make one though. I am trying to pick apart BigInt.js (the v8 bigint.cc is too large and I'm not familiar with C++).

For myself and perhaps others in the future, could one explain what the data model looks like for a BigInt that supports arbitrary sized integers? Basically, what is the object and its properties. I get that there are all the arithmetic functions implemented in unique ways for the BigInt, but I don't see what the kernel is. What is the essence of the structure of the BigInt? Perhaps this one will be slightly easier to grok.

I posted a question here asking if it was possible to create a PRNG in which an asymmetric private key could advance the PRNG while the public key could only reverse the PRNG. DannyNiu suggested an approach using RSA keys, for which I am working on a proof of concept here. After advancing and reversing the PRNG using this method, I expected the starting and ending states of the PRNG to be identical, but in my implementation, they are not. What am I doing wrong?

In Java, I can get a BigInteger from a String like this:

I'm following The Coding Train and learning about the p5.js library. In this particular exercise, I want to work with potentially massive integers, so I looked around and found peterolson/BigInteger.js.

I attempted to include this script in my project in the same way the p5.js library itself is included, and as suggested in BigInteger's readme:

Let's think about the following situation.

The Go routine creates a byte array where packs a Uint64 number 5577006791947779410 in 8 bytes Big Endian [77, 101, 130, 33, 7, 252, 253, 82].

In JavaScript code I receive these bytes as Uint8Array. We know that JavaScript doesn't currently support Uint64 as safe numeric type and cannot perform bitwise operations on integers larger than 32 bits, so things like buf[0] << 56 will never work.

So what is the process of decoding these bytes directly to numeric string "5577006791947779410"?

P.S. I know there are plenty of libraries for working with big integers in JavaScript, but generally they are huge and provide lots of mathematical operations, which I don't need here. I am looking for a simple modern straightforward solution for just decoding BE-packed Uint64 and Int64 bytes to numeric string. Do you have anything in mind?

I made a Java program that gives me desired output using Java's BigInteger Class.