Question about Transcend JetDrive 855

That is the whole purpose of this product. The enclosure and the SSD inside it has this non-standard connector system that Apple uses that is functionally the same but the pins are reversed as opposed to the usual M2 connectors. This means that this SSD goes into the Mac and the original one in the Mac goes into the enclosure and can still be used.
However, the controller in the enclosure is extremely slow and while the SSD connected internally has a throughput of more than 1.5 GByte/s, the throughput via this enclosure is around 240 MByte/s. So install it internally! So install it internally!

But if you want to skip all this monkey business, special prices for fantasy connectors and mediocre performance, buy an adapter from normal M2 to this Apple nonsense and a Samsung 970 EVO. Much better price, much better performance.
Just google the adapter. For example, "Sintech NGFF M.2 nVME SSD Adapter" costs €16 on Amazon.