where to find el capitan to upgrade

If you do have an operating system on your mac and it is functioning then you can download

El Capitan using this method.

Open your Safari browser and click on the link below,

How to get old versions of macOS – Apple Support

Go to Download OS and click on OS X El Capitan 10.11

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan.

This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the 

InstallMacOSX.dmg if you need it in future, or you could just make a copy of the Install OS X El Capitan.app 

prior to installing and moving it to an external drive for safe keeping. 

It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app in the Applications 

folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

Back up your data, check your applications for compatibility, and click on El Capitan. Older OS versions won't appear in a search of the Mac App Store.

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What does this paragraph in bold text from my earlier post say.

The next section can only be done on a mac that is capable of running El Capitan.

This includes macs that have the potential to run El Capitan but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

From the above paragraph you can conclude that a MacBook Pro from 2019 which came preinstalled with a

later OS than Mac OS X El Capitan will refuse to do the conversion from InstallOSX.pkg to Install OS X El Capitan.

If you cannot source a suitable mac that will enable you to do this then you can try this workaround on your

2019 MacBook Pro,

https://apple.stackexchange.com/questions/379603/how-do-i-create-el-capitan-installer-on-a-catalina-or-post-el-capitan-installe

to upgrade to a newer version of macOS go into the App Store and search macOS then you should be able to download it from there

And how do I convert the El Capitan .pkg to an .app when my new MacBook 2019 Pro won't do it? 😣 I need to create a bootable USB for my old iMac late 2009.

To convert the pkg to an app you just need open and run the pkg and it will automatically convert to .app