How does steam prevent oxidation in tea leaves?
According to Tony Gebely at Tea Epicure (https://teaepicure.com/tea-leaves-oxidation/), it's not steam per se, but rather heating that slows oxidation, by denaturing two enzymes (polyphenol oxidase and peroxidase) that are responsible for it.  Steaming is simply one way to do this:

Much of the oxidation process revolves around polyphenols, particularly the enzymes polyphenol oxidase and peroxidase. When the cells inside tea leaves are damaged and the components inside mix and are exposed to oxygen, a chemical reaction begins. [Specifically when polyphenols in the cell’s vacuoles and the peroxidase in the cell’s peroxisomes mix with polyphenol oxidase in the cell’s cytoplasm. From Michael Harney’s 2008, “The Harney and Sons Guide to Tea.”] This reaction converts the polyphenols known as catechins into flavanoids called theaflavins and thearubigins (which are also polyphenols).


Fixing works by denaturing polyphenol oxidase and peroxidase – the enzymes primarily responsible for oxidation using heat. Fixing is also commonly referred to as de-enzyming, denaturing or kill-green.


The term kill-green is derived from the Chinese term shaqing (杀青), which translates to killing the green. The tea leaves must be heated to approximately 150 degrees Fahrenheit to “halt” oxidation. Oxidation is further slowed by drying the leaves, but it never completely stops. At temperatures over 150 degrees Fahrenheit, oxidation continues to occur at an extremely slow pace.


The process of fixing requires precise control of the temperature and length of heating; each has to be adjusted depending on the size and thickness of the leaves and the amount being processed.

Note also that, for teas, oxidation is not a bad thing.  Rather it is a parameter that is precisely controlled in order to achieve the desired type of tea.  Green teas are produced by minimizing oxidation. Oolongs are produced by allowing oxidation to proceed part-way (and they are thus known as &quot;semi-oxidized teas&quot;).  Black teas are produced by allowing oxidation to proceed completely, or nearly so:

Oxidation refers to a series of chemical reactions that result in the browning of tea leaves and the production of flavor and aroma compounds in finished teas. Depending on the type of tea being made, oxidation is either prevented altogether or deliberately initiated, controlled and then stopped. [emphasis mine]


When oxidation is prevented altogether, the catechins are left largely intact. The tea leaves keep their green color and vegetal characteristics.


When a semi-oxidized tea is being produced, some catechins convert to theaflavins and thearubigins, resulting in a slight browning usually along the edges of the leaves and yellower liquor. Lipids, amino acids, and carotenoids also begin to break down into flavor and aroma compounds.


When oxidation is allowed to run its course, the leaves exhibit an aroma and taste profile completely unrecognizable from a finished tea that was exempted from oxidation.