China is only interested in providing loans to countries that has harbours, or other convertible assets. I am sure they are willing to finance a canal parallell to the Bosphorus, if they can collect the shipping fees for the next 100-200 years.
That would be possible only if Turkey promised utilization guarantee, like they did with the Japanese with their new Istanbul bridge.
The bridge is said to be badly under-utilized, but the government has given a certain rate of utilization guarantee for some period (30 years ?). By then, the investor will have had recovered their investment and then earn some extra. Then, the government will take over the asset (and then they may re-sell to another interest).
Now, they say the passing fee on that bridge is extremely high and utilization is very low. So, the government pays the constructor huge sums every month. It is, in my opinion, simply bad business mind. But, I guess, what matters is to make the relatives and friends of the president rich. The hefty sums are now being paid from the tax payers' money, regardless they use the bridge or not.
The canal project is an extremely expensive one and I believe its utilization will be much less than the new bridge. I do not even consider now-unknown climatic and geological side effects or the effects on the sea currents and biological life.
I think China gives money and is only interested about the return under international trade rules. How the Turks use it (probably badly) depends on them. China cannot be interested in a "build operate transfer" scheme unless there is no utilization and profit guarantee.