Starlink (the commercial company) has
re-applied to have 30,000 (additional?) Low Earth Orbit (LEO) satellites. Last I paid attention, I think they have on the order of 6,000 in orbit, and are approved for 12,000. But that's just the start.
You're probably used to thinking of how planes fly at different altitudes. Those going in particular directions are at a set of altitudes while those going in a different direction are flying at a different set of altitudes. (Assume a flat, infinite earth for a moment) Imagine if you had 10,000 planes flying east to west at one set of altitudes, and another set of 10,000 planes flying north to south at a different set of altitudes. If you keep the members of each set of planes separated from each other by, say, 1,000' feet, you don't have to worry about collisions between the two sets (except when they have to land).
The LEO satellites are deployed in orbits in shells, where a shell is a particular altitude (say 350 miles) and a particular angle (say 53 degrees) relative to earth's axis. Now suppose you put up 10 satellites in exactly the same orbit (ie, on an ellipse in a plane that cuts through the center of the earth) where each follows the other by 10% of the length of that orbit, meaning each follows the other 9(?) minutes behind it. Those satellites would never collide with each other. Now realize that the earth is rotating every 24 hours. Suppose at midnight, that first plane cut through the earth on a line from, say, EL Paso to Seattle. 10 minutes later, that plane is cutting through some large distance west of those two cities. Consider a second plane that cuts through El Paso to Seattle at 12:10AM. You could put up another set of satellites that share the orbit in that plane, without them crossing paths with those in the same shell but another plane. (I'm not an expert in this field so I may be inaccurate, but this is how I understand it.) For some visualizations, try this video
https://www.youtube.com/watch?v=VIQr1UyhwWk
By putting the shells at different altitudes, the satellites in one shell won't interact with another shell. By using a single angle within the shell, and using these rotated planes, one can pack a lot of satellites into a single shell. I believe Starlink is using (or has plans to use) three shells. The original shell used something like 53 degrees, which allowed it to cover most of the populated areas of the world. The second shell uses something approaching 90 degrees (ie, polar orbit) to cover the polar regions.
Starshield is the name for the US military set of LEO satellites. SpaceX is putting them in orbit (
Wikipedia indicates at least 183 in orbit by April 2025). AIUI, these would be in a different shell than those of the other LEO systems. (As GK3 wrote, there are a number of LEO systems being deployed, but Starlink is far in advance.)
And, as indicated in the article, China is putting up their own set of LEO satellites,
Guowang. I don't know if they have a second LEO network planned for the military or whether the satellites will be used for both commercial and military.
One can imagine that Starshield will employ military-grade communications as well as other military technology. Starshield would be the first target for China should we ever get into a kinetic war. But, even short of such a war, each side will be very exposed to the other side's LEO satellites.
By the way, LEO satellites are designed to eventually fall back into the earth's atmosphere and burn up. Normally, that will happen under the control of the operator, but if control of a satellite is lost, it will de-orbit within a few years.
Another use of LEO satellites is to reduce dependence on fiber-optic cables. For instance, should some enemy destroy all the fiber-optic cables between Hawaii and mainland US, satellite links can be used. Starlink has Inter-Satellite Links (laser communications) between their satellites. They speak of them being used in a mesh-like pattern. Other satellite deployments (middle-earth-orbit, geostationary) can also be used for this purpose, with higher latency.
I hadn't seen this article before:
Russia illegally using Musk’s Starlink satellites to advance in Ukraine
Since the Starlink User Terminals are licensed individually, and since they need to specify their approximate positions, I'd think Starlink can shut these down (even if a Russian fraudulent Starlink user terminal tries to identify itself as a licensed Ukrainian user terminal that is 1 mile away). If not, they didn't design their authentication well.