2015 MacBook Pro won’t boot to options using usb keyboard.

"2015 MacBook Pro won’t boot to options using usb keyboard.: [...]The keyboard and mouse work fine when putting in my password and operating the system, but won’t recognize during boot."

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Troubleshooting Mac TrackPad:

A. Reset the SMC and your NVRAM:

(Applies to macOS-Installed Macs only). Sometimes when changes are made to the system(i.e. updates), system configurations (i.e. for keyboard) become manipulated, technically. So, reset the SMC and NVRAM.

B. Boot into Safe Mode:

When you boot into Safe Mode, items load by default, with certain settings and caches set aside. So, hold down the shift key upon boot. See what happens then.

C. Single-Out this User:

What happens when you create a new Administrator user? do you still get this error? If not, then it it like an issue with the configurations of the current user. If so, then it is an issue of the macOS, in which case, you'd need to reinstall it.

D. Restore from a Backup:

Any recent Backups?: If you backed up your Mac prior to updating it then, restore form it. Sometimes, an update can fail due to power issues, or losing connection to Apple's servers. So, know to always back up your Mac prior to making any major changes to it, successfully as au update. In doing so, you with have something to restore store your Mac from from, should anything go wrong with the upgrade.

E. Reinstalling the macOS:

Another option would be to reinstall the macOS. Perhaps an install was not 100% successful? Sometimes, when major changes are made to your Mac(i.e. update installations), system settings (i.e. for the fan) become manipulated, technically. So, when reinstalling, nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to Reinstall macOS from macOS Recovery - Apple Support

Important: Be certain you back up your Mac prior to the installation, as I mentioned in C.

“Thank you for all the suggestions, but the system won’t respond to the keyboard. I’ve tried everything, but no luck at all. If I could get it to boot to install usb, I’d reinstall afresh and maybe sell laptop cheap as is. But while I can’t reinstall, I can’t do anything.”

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Use a Magic Keyboard:

Just curious: Are you using a third-party keyboard? You need the correct keys, and if without them (option and command), it may just not work. So, try an Apple brand keyboard (Magic). If you are already using one, then use a different keyboard —perhaps a friend or co-worker would have one for you to use?

Also, the SMC and NVRAM resets should each be performed 5 times if one does not work. It is what is recommended if just one reset will not do the job.

The Alt/Option key is the one most transposed on Windows keyboards.

Pretend the Windows keyboard has no labels on the keys, and try holding the key in the same position as Alt/Option on the Mac keyboard. That would be second from left on bottom row.

Be certain to press and hold the key(s) as soon as possible start startup