How do I make MAT the opening page ? Second question. How do I return to MAT after using one of the media programs I have tied? Thanks for your help.
What is MAT? 
A web application running on a regular web server, reachable via sub path like my.domain.org/mat and you want it to show on my.domain.org without path element?
Hmm, but that is a CLI+GUI toolkit, no web application at all, so in this case I wouldn’t understand the questions 
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My mistake. It is MATE. I have downloaded it from the list of programs in Dietpi setup. It is a desktop application
I assume I can open and run programs I have selected from the list of programs from my Pi 4 once dietpi opens?
DietPi is the operating system and not an application. You can’t run DietPi separately as it is already running 
To start Mate, you can type startx or select it as autostart option and reboot your system. This way Mate will be started automatically.
Thank you. Startx opened the desktop. Two more questions. I need to log in to fully utilize a downloaded program. It asks for a e-mail address. The e-mail address requires a @. My password also includes symbols. I am using a standard keyboard and when I try to add a symbol it results in a different result. @ becomes ". Same changes with others. How do I produce the required symbol?
Under Applications -Sound and Video only one of the three programs I downloaded shows. I have not been able to locate the other two. Any suggestions on resolving the two questions will be much appreciated.
Keyboard layout is set to UK by default. If you used a different layout, you would need to adjust this setting by using dietpi-config.
What apps you installed? I recommend to have a look at our online docs. There we describe all apps and how to access them.
Thanks again for the help. The two programs I installed and can’t locate are LMS and Squeezelite.
Did you checked our online docs as requested? LMS is a web interface and can be reached from every web server. It’s even possible from your local desktop computer.