lets say usa dollar and all currencies will be highly devalued .....then how we know the btc value??
The simple answer is we can't know that
It is not the devaluation of the currency that matters here but rather economic conditions in which it occurs. For example, if the world economy implode, it is unlikely that Bitcoin will be more worth in real terms. On the other hand, if inflation actually contributes to the economic growth (this is possible), its value may in fact rise both in real and relative terms (i.e. how much 1 bitcoin is worth in dollars)
most traders use USDT so if something happens with usa dollar is my USDT Will be worthless over night?
If the company behind USDT pulls an exit scam, this is a certainty