usdt tether market cap grows from 4,6 billions to 10,800 billions from March 2020
For last week - when btc jumped from 9 000 to 11 000 - tether issued 800 millions usdt
Do you think tether really has fiat for backing this usdt?

Currently they have not been able to prove it yet, there have been many accusations about USDT that they do not have the Fiat amount corresponding to USDT being circulated. If it is true, it is likely that USDT will collapse in the future and cause many people to lose money
USDT will never be able to collapse because basically this coin has a very important role in the crypto market, and almost all exchanges have this trading pair. There are many allegations about Tether but this stable coin still stands in the market, and no one is disappointed to use. If USDT disappears then perhaps everyone in this market will lose money because Bitcoin and USDT are often interlinked and will be very unlikely.