Why does my MacBook Air keep losing WiFi while being used, my IPad doesn't lose WiFi in at all.

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

    Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences.
6. Start the computer in Safe Mode. Test in safe mode to see if the problem persists, then restart normally.
7. Use Apple Hardware Test to see if there is any hardware malfunction.
8. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
9. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
10. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
11. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.6 Combo Update or Get MacOS Mojave now from the Mac App Store as needed.
12. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
13. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

     1. Select Disk Utility from the Utility Menu and click on the Continue button.

     2. When Disk Utility loads select the drive (out-dented entry) from the Device list.

     3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.

     4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)

     5. Click on the Apply button, then wait for the Done button to activate and click on it.

     6. Quit Disk Utility and return to the Utility Menu.

     7. Select Reinstall OS X and click on the Continue button.

Possible Fix for Wi-Fi Connectivity

First, see this article for possible solutions: How to troubleshoot Wi-Fi connectivity. If this doesn’t help then try the following:

1. Shut down the modem, router, and computer.
2. Wait at least one minute. Then in order:
3. Turn on the modem and wait until the connection is established.
4. Turn on the router and wait for it to connect to the modem.
5. Last, turn on the computer and wait until it connects with the router.

Tesserax (to Ryan Fiorini) 2 minutes ago

I am assuming that there must be some command file that is corrupted...

You may be onto something here...

Try the following on your MacBook, in order, until (hopefully) resolved:

Create a New User Account

1. System Preferences > Accounts
2. Click "+" to add a new account.
3. Log off, and then, log back on with the new user account.
4. Try connecting to a Wi-Fi network with your Mac's AirPort. See if it stays connected.
5. If this resolves the problem, there may be something amiss with your current profile and will require further troubleshooting.
6. If this does not resolve the problem, go ahead and log back into your normal account and go on to the next step.

Delete AirPort Keychain Entries

1. Launch the "Keychain Access" application located in Applications/Utilities.
2. In the windows on the left side: Select login for Keychains and "Passwords" for Category.
3. Click on the "Kind" filter at the top, and look for any "AirPort network password" entries...and delete them.
4. Close Keychain Access.

Delete & Add Back Preferred Network(s)

1. System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab
2. Delete all entries under "Preferred Networks."
3. Click on OK.

System Preferences > Network > Wi-Fi > Advanced > Wi-Fi tab

1. Add the preferred network(s) using the "+" button.
2. Click on OK.
3. Restart or log out, and then, back in.

If it works, then you are done.

If not, go on to the next step.

Move System Configuration Files

1. Quit any applications that are currently using Wi-Fi.
2. Turn off Wi-Fi from the Wi-Fi icon on the macOS / OS X menu bar.
3. Open Finder > Go > Go To Folder > Enter "/Library/Preferences/SystemConfiguration/" into the Go to the folder window > Click on Go
4. Locate and select the following files:

Note: Files are macOS / OS X version specific, not all the following files are used in each version.

• com.apple.airport.preferences.plist
• com.apple.network.eapolclient.configuration.plist
• com.apple.wifi.message-tracer.plist
• NetworkInterfaces.plist
• preferences.plist

1. Move any matching files to the macOS / OS X Desktop. You can place them all in a folder as a backup. The "missing" files will be rebuilt after restarting the Mac.
2. Reboot the Mac.
3. Re-enable Wi-Fi from the Wi-Fi icon on the macOS / OS X menu bar.

See these articles https://support.apple.com/en-in/guide/aputility/if-you-cant-choose-a-wi-fi-network-aprt2732/mac

https://support.apple.com/en-us/HT203147

Also make sure you are always using secured network settings WPA2 personal AES , set up google DNS servers 8.8.8.8 and 8.8.4.4 and no proxies are to be saved , location of network is to be automatic see this https://support.apple.com/en-in/HT202663

, also you must always create a password in the isp router , and its internal settings are not be disturbed it should be automatic .

Renew DHCP is to be applied see this also https://support.apple.com/en-in/HT202663

Note : A slight cut in the wires from router to Mac , loose connections or the wires are damaged could also drop wifi connections .

Continued from above:

14. If none of the above helps then see How to Downgrade macOS High Sierra and macOS Reversion- 

      How to Downgrade from High Sierra.

15. If you get here without success then it's time to make an appointment at the Apple Genius Bar for 

      service. If you need to find an Apple Store - Find a Store - Apple.

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