Can we somehow connect mobile device with our pc and use pc's ram, cpu etc. on connected mobile device or is there maybe a powerful external ram and processors for mobile devices?
Let us understand what RAM which is more commonly called Primary Memory in academics actually is. While you are computing in your processor those fast additions and substractions you need to store your inputs and outputs some where for that purpose processors have very small memory bank called registers which are along with processors and are quite fast but your inputs usually come from your storage i.e., a flash drive or a hard disk or something. First thing you need to know about these hard or even flash drives is they are very slow compared to your RAM which is nano seconds while your storage deals in microseconds.
Now bringing your memory in to RAM and computing in ram is helpful because all the memory operations now take nanoseconds. Now let us assume a remote RAM which is connected over WiFi may be. If you have 100Mbps connection to your RAM it takes 10 nano seconds for a single bit to get to your network port. Now you have to transfer it to your cache without loading it into any of the higher memory such as drives including ram, how do you plan to do that? I leave it here for you to brainstorm on these questions. A good exercise will be to look all the memory speed, device a design for your Remote RAM and calculate the latancies you might get and is it feasible to have those latancies at low level such as do a bit operation.
EDIT : Note the memory heirarchy include L1, L2 and L3 caches and Primary Memory. Things get interesting once you know about TLB's and stuff keeps digging.