hey so I pre ordered Call of Duty back in august and never received my beta code for it. Can ...

If you preordered digitally you will not receive code. You'll automatically be registered for the beta. Otherwise, you would need to contact the retailer you purchased the game from.

I pre ordered Marvel's Avengers from the Xbox Store on my console. Online it says the closed beta is on 14th August - 16th August, for people that have pre ordered the game. I have not received a beta code even though i have pre ordered the game.
Can you please give some advice on what I should do or when i will be able to play the beta?

You won't receive a code. This is from the Black Ops site:

I’ve pre-purchased my copy of Call of Duty: Black Ops III through the PlayStation Store, Xbox Live Store, or Steam, but I didn't receive a Call of Duty: Black Ops III Beta code. Where can I locate my code?

You don't need one! Code redemption is not necessary for pre-purchases through the PlayStation Store, Xbox Live Store, or Steam. You are automatically validated to participate in the Call of Duty: Black Ops III Beta.

If you purchased it digitally then you dont get a beta code as you dont need one. You are automatically added to the beta when purchasing it digitally through Xbox Live or PSN.

4. I pre-ordered Call of Duty: Black Ops 4 digitally, but didn't receive a Beta code. Where can I find it?

You don't need one! Code redemption is not necessary for pre-orders through the PlayStation Store or Microsoft Store, or pre-purchases through Battle.net. You're automatically registered to participate in the Call of Duty: Black Ops 4 Private Blackout Beta
on PlayStation 4 or Xbox One or Early Access to the Blackout Open Beta on PC. When the Blackout Beta begins, you'll be able to download it immediately.

Source:

https://support.activision.com/articles/en_US/FAQ/Call-of-Duty-Black-Ops-4-Beta

Hey Pred,

I'm not too sure where cullman man has been getting his Call of Duty news from but I'd advise him to look elsewhere moving forward. While no date for the beta on Xbox One and PC has been confirmed, the PlayStation 4 beta begins on August 19th and it's safe
to assume that it will come to Xbox One shortly after.

If you've pre-ordered the game digitally, you're set and the beta will be added to your account upon its release. If you've pre-ordered with a participating retailer, they'll have sent you a code for redemption at callofduty.com/beta. Once this has been
redeemed you'll receive a 25 digit code for redemption on Xbox LIVE in an email shortly before the beta begins.