What is the correct way to boil spinach?

The correct way to boil spinach, Japanese mustard spinach, etc.

Plenty of boiled water. (5 times the amount of ingredients)
Put in boiling water. (to shorten the boiling time)
Add 2% salt. (sometimes baking soda is added)
Do not cover.
When it is boiled, immediately cool it with water.

(1) and (2) are to prevent the temperature of the pot from dropping when the vegetables are added, which will lengthen the boiling time. The longer the boiling time, the less chlorophyll and the more pheophytin (yellow), resulting in a worse color. On the other hand, at 70°C, an enzyme that converts chlorophyll into green chlorophyllin kicks in. In other words, in order not to make pheophytin, boil it in plenty of boiling water to quickly bring it to around 70°C.

③ is to prevent fading of vegetables by adding salt. (If you add baking soda, the boiling water will become alkaline, and while it will turn dark green, it will affect the taste and texture.)

④ is to discharge the organic acid eluted from the vegetables to the outside of the pot together with the steam. (If organic acids are eluted into the boiling juice, the pH will drop and the color will deteriorate.)

⑤ is to prevent browning due to residual heat by soaking in cold water after heating. (However, water-soluble components will elute, so take them out as soon as the temperature drops.)

Miso soup and mashed soup that use green and yellow vegetables should be prepared just before serving.
Even in soy sauce or miso soup (when the pH is low), if the temperature is 60°C or less, adding green and yellow vegetables (boiled) makes little difference.

From here on it’s subjective.

When boiling leafy vegetables, the speed at which the stems and leaves are cooked is different. Therefore, it is said that it is better to put the root part first when putting it in the pot, and to match the timing of boiling with the leaf part.