For the 70% figure I’d have to dig a bit to fully check current volume. It’s been widely repeated in articles, they use data which check flows through exchanges.
You're talking about spot trading volume on centralized exchanges, which is a completely different metric than "transactions" (regardless of if you want to interpret that as on-chain or not).
Separately, Binance is a lot of bot-trading, and, outside of BN, USDT-heavy exchanges have questionable volume figures (wash trading and fake trades).
USDT usage has indeed gone down in favor of other stable coins, and while Binance is huge among exchanges, it's just one fraction of the BTC economy.
As I mentioned, I think it's too ambiguous to talk about "transactions" if we want to compare something measurable - if you want to restrict yourself to on-chain transaction numbers or volume, you'd have to just look at the on-chain activity - I am just assuming BTC is dwarfing USDT here, but that's just a small part of the story. On-chain activity is also meaningless for what I guess you're after, since there's no reliable way to separate self-transfers from other transactions.
If we go by what your sources allude to - trades on centralized exchanges - it'd be a matter of summing up the trading volume for each par involving either asset on any side.
Let's first look at Binance for the past 24h. I get:
BTC: 1.9 BUSD
USDT: 3.8 BUSD
Not so far off from the 70/30 number. This is not so surprising though, as USDT is a popular base-pair on Binance - but this is way different than saying that Bitcoin rests on USDT. There are other stablecoins on Binance, and if trouble or further doubts of confidence comes to Tether, liquidity will migrate fast. Indeed, it already is, gradually.
If we look at more exchanges (here 31 in total):
BTC: 7.6 BUSD
USDT: 9.6 BUSD
In either way, claiming that Binance or even the sum of all exchanges represent the whole bitcoin economy is ludicruous. Consider that USDT is almost only used for off-chain trading on exchanges, where Bitcoin is transacted in a lot of other ways.
OTC transactions (even those run by exchanges, like Coinbase) are not included here, for example. Neither are payments, on-chain transactions, or L2. Some will argue (I don't) that derivatives markets like BitMEX (margined/settled in BTC) and CME (margined/settled in USD) are reasonable to include as well, which are huge in BTC and again negligible in USDT.
As for sources - any serious exchange provide APIs for trades, and there are vendors that aggregate them. Coindesk and Coinlib acquire APIs and data from such vendors, who base them on the self-reporting of exchanges.
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I don't think there's any published recent study that looks at this properly. It takes effort or money to get the proper data, knowledge to model it, and time to compile it. Most of the people I know with the means are having their hands full with other stuff right now ;)
Source? Because this is very far from any data I have.