I'm trying to replicate for another region.

I put shp files in a directory on my computer, and adjusted the list.files call in the get)data() function appropriately, but needed to adjust the pattern for it to return the appropriate file.
using here:

library(here)

This didn't appear to work:
filenames <- list.files(path = paste0(here(), "/HydroRIVERS_v10_au_shp"), pattern="*.shp", full.names=TRUE)
This did:
filenames <- list.files(path = paste0(here(), "/HydroRIVERS_v10_au_shp"), pattern="\\.shp", full.names=TRUE)

Probably should use a $ at the end of the pattern as well:
https://stackoverflow.com/a/4876869/4927395`

This could have been a problem at my end though.

I think this should be the correct link?
https://milospopovic.net/map-rivers-with-sf-and-ggplot2-in-r/

I've modified my own get_data() as below:

# Download manually from https://www.hydrosheds.org/products/hydrorivers HydroRIVERS_v10_eu_shp.zip
# or use wget in terminal
# wget https://data.hydrosheds.org/file/HydroRIVERS/HydroRIVERS_v10_eu_shp.zip

get_data <- function(url, res, filenames) {
  #unzip("HydroRIVERS_v10_eu_shp.zip") #unzip, need once only
  filenames <- list.files("HydroRIVERS_v10_eu_shp", pattern="*.shp", full.names=T)
  return(filenames)
}

On a linux machine, comment out this:`

#windowsFonts(georg = windowsFont('Georgia'))`

modify make_map.r: by commenting out text...:

     theme(#text = element_text(family = "georg"),

And then make sure (in mainr.r) you source your local make_map.r:

# map_url <- "https://raw.githubusercontent.com/milos-agathon/map-rivers-with-sf-and-ggplot2-in-R/main/R/make_map.r"
# source(map_url) # load script
source("R/make_map.r")