In RT-Thread Studio, now only imxrt1052 and 1064 are supported, but i.MX6 is supported by rt-thread os,you can use this bsp with scons and env toolsets
https://github.com/RT-Thread/rt-thread/tree/master/bsp/imx6ul

Different from PC OS，it's hard to describe the rt-thread's thread is a process or a sub-threads, these points may help you understand it:

1. different thread in rt-thread share the same memory space, so the global variable and other ram space are accessible between threads
2. each thread has its own context, so the general registers, local variables, are separated