I am new in the gaming industry so I want to build a gaming pc that is under 300 can you any suggest?
I find these links on google

https://www.wepc.com/builds/best-gaming-pc-under-300/
https://gurubuilds.com/best-gaming-p...r-300-dollars/

Okay: the title of your post poses one question, but the body poses a different question.
No, a $300 gaming PC is not going to be "best" by most measures for playing AAA games. (Exception, a whole HERD of AAA DOS games would fly on any modern machine, but I presume you mean CURRENT games). Someone with a strong tech background and A+ certification or equivalent should be able to build an excellent gaming PC for less than the $1500 to $4600 or more it might cost to BUY one pre-assembled from someplace like DELL, but it is unclear that you are asking about that.

Check out pages like https://www.pcmag.com/picks/the-best-gaming-desktops for expected costs for a pre-built.

If I understand you correctly and you really want to build your own to get the most game experience for the $ (with a $300 limit), then it becomes important for someone to ask "how much PC build experience do you have?". What parts have you on-hand and what will you need to source?

Whenever I see posts like this I get a little skeptical, and sometimes I like to fire up my Tor Browser and check these links.
My skepticism was not confirmed this time, but the first link (wepc.com) is so chock full of third-party scripts, I really do not recommend clicking that.
Gurubuilds seems kinda OK though.

Ok, for example Far Cry 6 is an AAA game based on this site: https://www.g2a.com/news/features/best-aaa-games/ (and not the very first one).
The [official] recommended hardware is:
At the end it is definitely much more than $300.

^ Yes, I had noticed that the catchy title melts away when you actually click the Amazon links and see what each "gaming rig" consists of: yesteryear's AMD CPU with integrated graphics, a smallish SSD, a cheap motherboard, not enough RAM...

Add $100 to the budget and get a Steam Deck.