example.com

a)

Instead of

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

Please try to add above htaccess code in reply1

Moreover, regarding the permalinks issue,
go to settings -> permalinks-> select defualt one and hit save.
and try to check the newly created website in new incognitio mode window or different browser.

Update:
Solution posted in reply no.1 part (i) have a problem, which is
example.com would be opening without https as well and not redirecting.

Solution: Add following lines to .htaccess

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} ^firemudfm\.com$
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://firemudfm.com/$1 [R,L]
</IfModule>

Assuming you are running 4.0+ wordpress version,
Well, According to WordPress Codex
https://codex.www.ads-software.com/Multisite_Network_Administration
If your wordpress version is 3.5+

then following should be part of .htaccess

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ wp/$1 [L]
RewriteRule . index.php [L]

it is suggested to try aforementioned htaccess config

Update: Following error is
b) Not Solved:
From https://example.com/wp-admin/network/sites.php
when clicking on Dashboard under https://multisite1.example.com
it opens https://multisite1.example.com/wp-admin instead of https://multisite1.example.com/wp-admin

Checked: siteul and home is set to https://multisite1.example.com for network site…

Anyone have idea to avoid this redirect

Update:
a) Solved: https://multisite1.example.com by updating following things->
i) Removed following lines .htaccess

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://example.com/$1 [R,L]
</IfModule>

ii) In wp-config.php removed

define('FORCE_SSL_ADMIN', true);
$_SERVER['SERVER_PORT'] = 443;

iii) mu-plugin for upgrade network error
Added a mu-plugin at wp-content/mu-plugins
_sslverify.php

<?php
add_filter('https_ssl_verify', '__return_false');
add_filter('https_local_ssl_verify', '__return_false');
?>