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SARS-CoV-2 is an enveloped virus: in structural diagrams it is drawn with membrane glycoprotein (M), envelope protein (E) and spike protein (S) embedded in a lipid membrane.

What specifically is the lipid membrane made of?

I assume it is derived (somehow) from the host cell membrane, and would have a similar composition. It doesn't necessarily have the same composition. If it just host membrane from a typical mammalian cell, it may be mostly phosphatidylcholine (PC) and cholesterol on the outside, and phosphatidylethanolamine (PE), PC and cholesterol on the inside. It should equilibrate over time, since the virus does not contain a flippase to keep an asymmetrical distribution.

Related questions: Does the composition depend on the cell that the virus was assembled in? Does it have any selectivity, or does it simply pick a random piece of the host membrane? How exactly is the viral membrane detached from the host?

Probably most important question: Does the infectivity of the virus depend on the composition of the membrane?

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    $\begingroup$ Structural protein (M, also S,E) have transmembrane domains anchored inside cell's Ergic ER and Golgi infection-modified vesicles, they'd take some lipids with them during the assembly of the virions (expression of M plus low amounts of others produces self-assemblying viral like particles), origin of those virion factories vesicles is unclear. $\endgroup$
    – reuns
    May 4, 2020 at 3:36
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    $\begingroup$ The last reference says it is the virion factory vesicle (1 2 3) which is exocytosed all at once. $\endgroup$
    – reuns
    May 4, 2020 at 3:46

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I don't know if this questions has been studied in detail even for SARS, let alone SARS-CoV-2. As noted in a comment by reuns, the virus lipid envelope is most likely derived from that of the endoplasmic reticulum.

What we know however from from some studies on influenza viruses, that's hardly the end of the story though:

Using rigorous virus purification, marked differences between virions and host were observed. Over 125 phospholipid species have been quantitated for three strains of influenza (HKx31- H3N2, PR8- H1N1, and VN1203- H5N1) grown in eggs. The glycerophospholipid composition of purified virions differs from that of the host or that of typical mammalian cells. Phosphatidylcholine is the major component in most mammalian cell membranes, while in purified virions phosphatidylethanolamine dominates. Due to its effects on membrane curvature, it is likely that the variations in its content are important to viral processing during infection.

And this part is a bit of a "reverse answer", but lipid rafts on the membrane of the target cells are thought to be important for the entry of coronaviruses, including SARS. One experiment found that depleting cholesterol from the cell membranes dramatically slowed SARS virus' ability to enter them.

(Regarding how SARS-CoV-2 might come out [with different envelopes] from different cells: we only had confirmation very recently that it substantially infects intestinal enterocytes. So expecting a lot of extra details on that is a bit premature right now.)

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