who determines which stock goes into SP500 and Nasdaq index

Who determines which stock goes into SP500 and Nasdaq index?

The S&P500 index is created by a division of Standard and Poor's Global. They use their own criteria to pick the index members. They periodically adjust the members of the index they create, they also make adjustments when companies merge, split, or go bankrupt.

The NASDAQ main criteria is that the stock has be sold on the NASDAQ stock market. The people that run the market, determine who is in the index.

Both of these indexes are not the only index that the company maintains. There are also funds that mimic these indexes.

Is it possible to get a notification when the decision has been made about adding/removing a stock from the index?

The people that create and maintain these indexes issue a press release whenever a change is made. The companies being added to the index also issue a press release. These two indices along with the Dow 30 are the three most watched indexes in the world, and they don't hide when they make a change.

From Wiki:

S&P Dow Jones Indices LLC is a joint venture between S&P Global, the CME Group, and News Corp that was announced in 2011 and later launched in 2012. It produces, maintains, licenses, and markets stock market indices as benchmarks and as the basis of investable products, such as exchange-traded funds (ETFs), mutual funds, and structured products.

The company's best known indices are the S&P 500 and the Dow Jones Industrial Average (DJIA), which was created in 1896. The company also manages the oldest index in use, the Dow Jones Transportation Index, created in 1882 by Charles Dow, the founder of The Wall Street Journal.

The NASDAQ is responsible for changes in the NASDAQ-100 (and other NASDAQ indexes). These occur whenever there is a delisting (merger, bankruptcy, failure to meet listing requirements) as well as at year end when the index may be rebalanced.