Upload External File to Drupal File System Situation

I would like to be able to upload files programmatically from an external source to the drupal file system. I was thinking of making a RESTful API that allows me to save external files into my site's public file path. The file is an excel file as well if that changes anything.
Problem

Currently, I am thinking of just using an S3 bucket to store the external files. Then within my site, I would simply run a GET REQUEST and programmatically save the file. I was just hoping there was a way to simply POST a file in drupal without me having to use Amazon's S3.
Parameters:


Drupal Version 8.8
Drupal Site BackEnd= PHP 7.3
External App FrontEnd = Vue.js
External App BackEnd = .NET Core
Required File Type: .xlxs
Secondary File Types: .csv, .docx, .json, .jpeg, .mp4, .txt

TL;DR

How can I make it so another site/app can upload files to my site? (using JavaScript)

Notice:
I understand StackOverflow is only meant for help with code issues but I would greatly appreciate some help getting started and if not at least some very useful links that are credible. Some of the research I have found is not useful seeing that I am on Drupal 8.8
Take a look at foldershare and foldershare_rest modules, they provide entire data management pipeline on a Drupal site, along with a ready to use rest client cum command line utility. Here are couple videos that show the module's capabilities

Managing data via REST client for automation and power users (5 mins)
Managing data on SeedMeLab for users(3mins)

Disclosure: I am the maintainer for these modules.