How Do I Create a Bootable Copy of El Capitan

My cousin has an old MacBook Pro (Late 2011, 15 inch) that was running El Capitan. The computer was not responding. She had nothing she needed on it so after checking the internet the best solution was to wipe the computer and start again. She had no time machine backups. I followed the instructions and wiped the computer. When I went to reinstall the operating system, El Captian came up as the only option, but I then got a Required Download is missing message. I discovered that Apple no longer supports El Capitan so that is probably the reason.

It appears to me that I need a bootable copy of El Capitan to revive her computer. I have been looking at this for hour with no success. El Capitan is not available on the App Store and I also tried the 'find the hidden app' method from YouTube. I believe this computer can support High Sierra which is available but I have to download it from my newer MacBook (2019 16" running Sonoma). There is no download option, but a "get" option which tells me the version is not compatible with my computer. So that was a dead end.

I've tried downloading High Sierra other ways but that gets the same result.

I also have an old MacBook that is running High Sierra so I tried using a Time Machine back up, but my cousin's machine is smaller so there is not enough space. I tried backing my old MacBook up leaving out all movies, music, pictures, etc. which should create a copy that is small enough to use on my cousin's computer but that also says there is not enough space.

I tried using terminal command I found on the web to get a bootable copy but they say 'not found'.

Get Help Online won't let me get to anywhere useful to download anything so that doesn't help.

Does anyone know where / how I can get a download of El Capitan (or High Sierra/ Sierra) that I can use to create a bootable copy? I am not particularly technical and just want a copy on a USB that I can plug into her computer to get it working again, no matter how basic.