Let's say I am bullish on "Cloud Computing" and I want to make one investment, but do not want to invest in any specific company (e.g. Amazon). How could I invest soundly in the "Cloud Computing" market, betting that this market will rise generally, while hedging against any individual company failing?
You need to hope that a fund exists targeting the particular market segment you are interested in. For example, searching for "cloud computing ETF" throws up one result.
You'd then need to read all the details of how it invests to figure out if that really matches up with what you want - there'll always be various trade-offs the fund manager has to make. For example, with this fund, one warning is that
this ETF makes allocations to larger firms that are involved in the cloud computing space but derive the majority of their revenues from other operations
Bear in mind that today's stock prices might have already priced in a lot of future growth in the sector. So you might only make money if the sector exceeds that predicted growth level (and vice versa, if it grows, but not that fast, you could lose money). If the sector grows exactly as predicted, stock prices might stay flat, though you'd still make a bit of money if they pay dividends.
Also, note that the expense ratios for specialist funds like this are often quite a bit higher than for "general market" funds. They are also likely to be traded less frequently, which will increase the "bid-ask" spread - i.e. the cost of buying into and getting out of these funds will be higher.