Error 404 on things after "/"

Despite all the information given by you suggests using xampp, as in the opening of the problem you informed: ""First, https://domain works. It calls the basic controller and method.", this led me to ask if there was a local site.
Assuming then that you are using xampp, allow me to suggest the following steps for solving the problem.
1 - in xampp/htdocs, create a folder, with the name you want, where all CI4 files will be moved;
2 - in xampp/apache/conf/extra, edit the httpd-vhosts.conf file which should look similar to the one below:

PHP Code:

# Virtual Hosts
#
# Required modules: mod_log_config

# If you want to maintain multiple domains/hostnames on your
# machine you can setup VirtualHost containers for them. Most configurations
# use only name-based virtual hosts so the server doesn't need to worry about
# IP addresses. This is indicated by the asterisks in the directives below.
#
# Please see the documentation at
# <URL:http://httpd.apache.org/docs/2.4/vhosts/>
# for further details before you try to setup virtual hosts.
#
# You may use the command line option '-S' to verify your virtual host
# configuration.

#
# Use name-based virtual hosting.
#
##NameVirtualHost *:80
#
# VirtualHost example:
# Almost any Apache directive may go into a VirtualHost container.
# The first VirtualHost section is used for all requests that do not
# match a ##ServerName or ##ServerAlias ??in any <VirtualHost> block.
#
##<VirtualHost *:80>
    ##ServerAdmin [email protected]
    ##DocumentRoot "C:/xampp/htdocs/dummy-host.example.com"
    ##ServerName dummy-host.example.com
    ##ServerAlias ??www.dummy-host.example.com
    ##ErrorLog "logs/dummy-host.example.com-error.log"
    ##CustomLog "logs/dummy-host.example.com-access.log" common
##</VirtualHost>

##<VirtualHost *:80>
    ##ServerAdmin [email protected]
    ##DocumentRoot "C:/xampp/htdocs/dummy-host2.example.com"
    ##ServerName dummy-host2.example.com
    ##ErrorLog "logs/dummy-host2.example.com-error.log"
    ##CustomLog "logs/dummy-host2.example.com-access.log" common
##</VirtualHost> 


you can remove # to enable the lines and make the necessary changes, but again let me suggest an example:
<VirtualHost *:80>
    DocumentRoot "C:/xampp/htdocs/ci-folder/public"
    ServerName localhost
    ErrorLog "logs/ci-folder-error.log"
</VirtualHost>

where: ci-folder is the name of the folder created in htdocs and where the files were moved to;
localhost must be informed in ServerName, because it works as an https certificate, browsers will recognize it as a "safe site", the alert "not secure" will not be displayed in the navigation bar;
finally, stop/start the apache server, most likely this will solve the problems.