How come big tech companies get away with stealing and modifying other people’s great ideas and incorporating them into their existing giant platforms thereby stifling the start-up with the original idea?

Why do we clap when this happen?

It is because an idea itself is worth nothing.

The first one to execute it well is most likely the winner

Well, the idea has to be patented first, and there are so many patents around... Big companies make a serious effort into acquiring them for this purpose. E.g. amazon owns the patent for a 'photo studio with lightning in the room'....

The patent system is corrupt and doesn’t help smaller businesses when mega corps steal their ideas. Smaller businesses do not have the capital to take them on legally. Mega corporations also have the funding to flood the market with advertising to kill off the original competitor even if they weren’t first to market. Just look at candy crush. The idea was stolen from an existing game. The guy who made it got a settlement out of it, but it was still theft.