winusbnet | WinUSB .NET wrapper library | Form library

Well, the weirdness you are seeing around 64 byte transfers is well known:

Delimiting write transfers with short packets

The USB driver stack driver does not impose the same restrictions on packet size, when writing to the device, that it imposes when reading from the device. Some client drivers must make frequent transmissions of small quantities of control data to manage their devices. It is impractical to restrict data transmissions to packets of uniform size in such cases. Therefore, the driver stack does not assign any special significance to packets of size less than the endpoint's maximum size during data writes. This allows a client driver to break a large transfer to the device into multiple URBs of any size less than or equal to the maximum.

The driver must either end the transmission by means of a packet of less than maximum size, or delimit the end of the transmission by means of a zero-length packet. The transmission is not complete until the driver sends a packet smaller than wMaxPacketSize. If the transfer size is an exact multiple of the maximum, the driver must send a zero-length delimiting packet to explicitly terminate the transfer

Delimiting the data transmission with zero-length packets, as required by the USB specification, is the responsibility of the client driver. The USB driver stack does not generate these packets automatically.

From USB Transfer and Packet Sizes on MSDN

As for the rest... depending on whether the device declares itself to be a composite device or not, Windows's driver loader will pick either one or multiple device drivers to connect to the device. Those drivers handle choosing which endpoint on the device to talk to. So from userspace, it is normally enough to open the driver interface and begin doing I/O. The HID class driver, for example, knows how to identify and enable the HID endpoint.

Since you have things working with UWP, it seems likely that you have the WinUSB driver loaded (since that's Step #1). Therefore you'll use the WinUSB API to talk to it.

Here's the documentation for the C and C++ API for WinUSB. It has examples of endpoint setup, and looks quite a bit less messy than the libusb code you quoted (although that may have to do with code formatting and style as well).