How to make multiple requests via one endpoint

I’m practicing as a front end dev and I should code opening .md files with use express node js. Also, in some .md files could be a links [[path-to-the-md-file]] like this and I should make them looks like this <a href='path-to-the-md-file.md'>link</a>. But I really don’t know how to make this. This is my app.js file:

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');

const indexRouter = require('./routes/index');
const usersRouter = require('./routes/users');
const serveMarkdownRouter = require('./routes/serveMarkdown'); 
const app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'twig');

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', indexRouter);
app.use('/users/:id', usersRouter);
app.use(function(req, res, next){
  if(!(String(req.url).includes('/serve-md/'))) res.render('error');
  else{
  const fs = require('fs');
  const parseMD = require('parse-md').default;
  var express = require('express');
  var filePath = 'posts/readme.md'; // default md file
  console.log(req.baseUrl);
  const fileContents = fs.readFileSync(filePath, 'utf8');
  var { metadata, content } = parseMD(fileContents);
  res.render('serve-markdown', {title: 'Serve-markdown', metatitle: metadata['title'], metadescription: metadata['description'], cont: content});
  }     
})
 
app.get('/', function(req, res){
  console.log("Render Working")
  res.send();
});

// app.use('/serve-md/:path', serveMarkdownRouter);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  next(createError(404));
});

// error handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};
  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

I know I could do this through some Router like this app.use('/serve-md', serveMarkdownRouter);, but I couldn’t handle all file links via this router. What should I read or do to make my task done? Thanks.