To my knowledge, no proof has actually been publicly presented for this claim. There have been a few stories that didn't pan out (like the one that boiled down to, "Huawei devices have telnet installed"), but no actual evidence of backdoors has come to light yet.

This is despite the fact that Huawei has been under an extraordinary level of scrutiny for years. British intelligence was given extensive access to Huawei's hardware and code, as a condition of Huawei equipment being installed in the UK. We know from Snowden that the NSA hacked into Huawei HQ, but there's no indication that they found any evidence of backdoors. And despite running a global campaign to convince/pressure other countries not to use Huawei, the US hasn't publicly unveiled any evidence of Huawei backdoors. British officials have even admitted that the UK's decision to ban Huawei was based on pressure from the US, not evidence of wrongdoing.[0,1] This all makes me think that the US, UK et al. don't actually have proof of backdoors.

0. https://www.theguardian.com/technology/2020/jul/18/pressure-...

1. https://www.euractiv.com/section/politics/short_news/uk-bann...