Create a folder C:\Esxibuild
Download to C:\Esxibuild
VMware vSphere Hypervisor (ESXi) 6.7U3b Bundle (not the ISO).
https://my.vmware.com/web/vmware/details?downloadGroup=ESXI67U3B&productId=742
Download the Realtek Drivers:
https://vibsdepot.v-front.de/wiki/index.php/List_of_currently_available_ESXi_packages
(For this one, I used the net55-r8168-8.045a-napi-offline_bundle)
Here is what the files look like:
Download and install the VMware PowerCLI
https://my.vmware.com/group/vmware/details?productId=614&downloadGroup=PCLI650R1
Open powershell. Got to the folder C:\Esxibuild in PS.
Add the VMware Modules for Powershell:
Get-Module -ListAvailable VM* | Import-Module
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Check Modules:
$env:PSModulePath.Split(';')
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Import Modules:
Get-Module -ListAvailable VM* | Import-Module
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Find the build information. Open the compressed file for the Esxi bundle you downloaded: c:\esxibuild > ESXi670-201912001 > metadata > profiles. There should be 4 profiles. Copy the path for the next commands.

Change your paths to match.
run:
Add-EsxSoftwareDepot "C:\esxibuild\net55-r8168-8.045a-napi-offline_bundle.zip", "C:\esxibuild\ESXi670-201912001.zip"
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Get-EsxImageProfile
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New-EsxImageProfile -CloneProfile ESXi-6.7.0-20191204001-standard -name ESXi-6.7.0-20191204001-standard-RTL8111 -Vendor Razz
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Set-EsxImageProfile -ImageProfile ESXi-6.7.0-20191204001-standard-RTL8111 -AcceptanceLevel CommunitySupported
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Get-EsxSoftwarePackage | Where {$_.Vendor -eq "Realtek"}
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Add-EsxSoftwarePackage -ImageProfile ESXi-6.7.0-20191204001-standard-RTL8111 -SoftwarePackage net55-r8168
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Export-EsxImageProfile -ImageProfile ESXi-6.7.0-20191204001-standard-RTL8111 -ExportToIso -filepath C:\esxibuild\VMware-ESXi-6.7.0-8169922-RTL8111.iso
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Now, there should be an ISO to burn.

Other sources: https://www.sysadminstories.com/2018/08/adding-realtek-8111-driver-to-vsphere.html