El Capitan full installer

You downloaded the correct full installer contained in the InstallMacOSX.dmg.

When you open that to the InstallMacOSX.pkg you then open that and you see an

installation window. This is where people get confused.

The installation window does not install El Capitan, if you read the text it says it

will take 7MBs of disk space, no operating system takes up so little room, and why

would it take up only 7MBs of space when the initial download was 6.2GBs.

By running the installation you are actually creating the Install OS X El Capitan.app

in your Applications folder. Have a look.

(It is also important to note that when the installation window asks

you to select a disk, you need to select the disk you are booted up to at the time

not any other internal or external disk you want to actually install El Capitan on,

that comes later when you are ready to actually install El Capitan.)

Now that you have the Install OS X El Capitan.app in your Applications folder

you are ready to install El Capitan.

The installation will take 25-30GBs of disk space and about 15-30 minutes to install.

Here is the official Apple article on how to create a bootable macOS USB installer:

https://support.apple.com/en-us/HT201372

You can use a bootable macOS USB installer on any Mac that is compatible with that particular version of macOS.

I have given you the full instructions above, have you read them.

As long as the "OS X El Capitan.app" is complete and there is no corruption, then it should be usable to make a USB installer. The .dmg file is not needed once the installer app is in the Applications folder. As @Eau Rouge mentions if you are downloading the El Capitan .dmg installer file there are several steps which must be done before the installer shows up in the Applications folder. The installer must be present in the Applications folder before a USB installer can be created.

Carefully read the Apple article you linked in your earlier post as it contains all the instructions needed to download and "install" the installer to the Applications folder. Then if you want to create a bootable macOS USB installer, then follow the instructions in the Apple article I provided previously. Keep in mind that Macs are very picky about the USB sticks used for booting so if you still have problems, the try using another brand of USB stick. The quality of USB sticks is extremely poor even from respected name brands.