Running games that you already have stored loacally.
There are 2 options that are best here and that is AEL or emulationstation.
This is the way i used to run my setup before i found IAGL.
I chose emulationstation as i had much knowledge of this as a long time user of retropie.
Either one of these frontends can be installed from the gamestarter repo.
So after i installed emulationstation the first thing i would do is configure the addon, to do this find the addon in your programs select it and select configure.
The first options that pop up allows you to download extra themes, you can go ahead and do this if you like. Again personal preference, i had one backup theme from the retropie days that i had edited to my liking so i just copied that over.
After this go down to es_systems here you select where your rom directory is and which cores you would like to use.
To get you started just select your rom directory and then select update the es_systems.cfg leave all the core settings unless you know what you are doing and which cores you actually have.
you must have you roms directory console names match the same as retropie so for example commodore 64 is "c64" nintendo is "nes" etc, Again do a little google research to find out how the naming of your roms directory should be.
With you preferred controller connected you can now launch emulationstation on first launch it will ask you to configure your controller, after that all games will show in emulationstation and you can just launch form there and have some retro fun
Again there are further things you can do to enhance your experience, like i had all my roms scraped images and metadata using universal xml scraper, then selecting a theme that has metadata gives you a amazing looking system with boxart, screenshots, description of the game etc but im not going into all of this as all im trying to do is get you all up and running.
Thats pretty much all i can tell you about getting games to run on libreelec, there is much more options you can adjust to your liking so if you come across anything your unsure of just give me a shout and hopefully i can help
Happy Retro Gaming