Wifi does not appear in interface pop up selection. How to fix?

One issue, prior to Big Sur was that the Apple WiFi kernel extensions (drivers) got messed up on Mojave / Catalina. It used to be possible to copy these kext files over and install them but not with Big Sur because the System files are so locked down the disk volume is triple protected. First with SIP, then with a Read Only APFS system volume and finally in Big Sur, Apple takes a snapshot of the system volume, signs and seals it and boots from the snapshot. So even if you could modify the read only system it's not actually being used to boot. Just the snapshot of it is used.

If all else fails, try booting into Recovery Mode and re-installing Big Sur over the top of your existing system. If that fails, backup everything and create a bootable USB flash drive and boot from that, completely nuke the internal disk and re-install from scratch. Then restore your data from backup.

You will probably need to do a clean install. Before that:

1) What do you see in System Information about wifi? Hold down option key and from the Apple menu choose System Information; then click Wifi. Post a screenshot.

2) Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.

Last chance thing I can think of is boot into Safe Mode to see if some third-party kernel extension is blocking it. Hold down the Shift key when restarting. If you have FileVault enabled, go ahead and hold down the shift key after entering your decryption password.

If it doesn't appear in Safe Mode, I would assume it isn't recognizing the hardware. If an SMC reset didn't fix the problem, you'll need to take it into an Apple Store or Authorized Repair Center.

First, try a creating a new "location". Does Wifi appear there?

If not, then reset NVRAM and SMC, and try again.

I don’t have the Etrecheck

So if you drop down the Interface list there is no WiFi listed?

I did reset. It won’t help. It’s all the same as before. The wifi interface isn’t visible. Is there any help I can get?

Etrecheck is a diagnostic utility by etresoft, a respected member of this community and frequent contributor.

You can download it from the website.

My Mac isn’t connected to wifi so, I can’t download it. Is there any other way to sort it out?

If you have access to another mac you can download and then copy it over.

Since we don't have access to your mac, Etrecheck is our best chance to know of possible causes of problems.