The single fact that you've been involved with Bitcoin since 2016 should make you know that Bitcoin is not backed by any currency. It is not backed by the dollar or any other currency. If the dollar collapses, the value of a bitcoin can simply be measured in another currency. It is mostly measured in dollars because that's the generally acceptable currency in the international market.

There are cryptocurrencies backed by dollars such as USDT and USDC. These are called stable currencies because they neither go above or below $1. They appreciate and depreciate based on the dollar. If the dollar crashes, they'll crash. That's a coin back by a dollar.
Gold, for example, is not backed by the dollar. If the dollar ceases to exist, gold would still have its value, only that it would be valued using other currencies in the international market. The currency that is used to put a price on the asset has nothing to do with the value of the asset.