Installing Mac OS on formatted drive

The Mac Startup Manager is a tool that allows you to choose different startup disks and access various boot options when your Mac starts up. You can access it by holding the Option key during startup on Intel-based Macs or by holding the power button on Apple silicon Macs until the startup options screen appears.

--this surprisingly accurate (but incomplete) summary was produced by AI search assist.

what they do not tell you is that to boot from a different drive on an Intel Mac after 2018 that has a T2 chip, you must adjust security options:

Install macOS on an external storage device and use it as a startup disk - Apple Support

look for the heading: Allow booting from external media

Is it an Intel Mac or an M-series Mac? This is very important. Since you mention using special startup keys & getting a -2100f error code, then I am assuming it is an Intel Mac.

If it is a 2018-2020 Intel MBPro, then did you modify the system security settings with the Startup Security Utility prior to erasing the internal SSD? If not, then you cannot use a macOS USB installer at this time since the default security settings won't allow it. And you cannot change those settings until you have macOS installed on the internal SSD.

You can try an SMC Reset (read the instructions since the process is different for a T2 Mac) and PRAM Reset (hold for two chimes if possible, but some of those T2 Macs won't let you get two chimes). I doubt this will make any difference.

You can also try a DFU Firmware Revive which resets the T2 security chip and system firmware.

How to revive or restore Mac firmware - Apple Support

If it is a 2017 or earlier MBPro, then it is possible to boot from a USB installer.

FYI, you can get the exact model of the Mac by entering the system serial number on the check cover page (please don't post the serial number on the forum since it is considered personal information):

View Coverage - AppleCare & Warranty

<< If I connect to the WiFi, it tries to start internet recovery, but gives a -2100F error code after a while (15 minutes?). I’ve tried on two different networks. >>

In Internet Recovery, these low negative thousands error messages indicate that your provided Wi-Fi Network is inadequate in some fashion, and cannot be used (for this purpose) in its current state. We have seen -1000 to -6000 reported, often with a letter appended.

Common problems include:

Hidden Network-name

required use of a proxy server to get Internet access

required use of a login page to get Internet access

Use of a login server or certificate to get Internet access

Use of PPPoE in the Mac to get Internet access (typically only applies to DSL)

Use of Fixed IP address rather than good old DHCP

Variance of the time by more than five minutes

Failure of Router to provide workable DNS server addresses, or providing 0.0.0.0 [may produce -2002f]

If you have an Ethernet port, you may in some cases be able to move your Mac close to the Router and connect to the Router using Ethernet.

The higher the number, the more obscure the problem.

If your battery was completely depleted, the Date&Time could have reverted to the default, which will not work. There is a terminal command to reset to current Internet Date&Time -- post back for assistance.