What will happen if Tech Giants Like Google, Microsoft, Amazon starts investing in Bitcoin or Altcoin Mining?
If they did it right (developing new generations of ASIC's and running them in places with low energy costs), the diff would probably rise even more than it already did, forcing some smaller miners and miners with relatively high energy rates to stop mining.
For the rest, not much would probably happen, unless they suddenly added a ton of hashrate to the network one week, and took everything offline a couple months later... In this case the average time between two blocks would first be smaller than 10 minutes untill the next readjustment, then when they took their farm offline it increase drastically untill the next readjustment (every 2016 blocks)
Yeah, that would be great to have big tech company join in bitcoin but China miners will still lead the mining due to their advantages in the country (hardware and low cost electricity).
I think so, it wouldn't/never be happen as they have their own project and interest. We just depends on developers to solve blockchain problems, to handle transaction issues and the block.