So if something happens with trust wallet company and their apps become inaccessible then how i will be able to retrieve my btc,dot and other blockchain assets by using that one multi wallet mnemonic phase?
In theory, at least the most known wallets can be restored from seed. That's in theory.
In reality, nobody could answer your question, nor should risk guessing. In the same way SBF had his backdoor in FTX accounting software, we just cannot know if this closed source wallet is nice and clean or not. In theory, it is, but that's only in theory.
And as long as trust wallet is closed source, whatever its developers (or Binance official) will tell ... you have to trust them and they may be lying or not. Even more, as long as they
keep lying it's open source, I choose not to believe them, I choose to use an actually open source wallet.