Would it actually be legal for the federal government of the United States to force the Chinese government to give up/sell Smithfield Farms to the US government or a US company if the US deemed it pertinent to "US national security"?
united states - Do governments need to compensate business owners if they temporarily force a business closure? - Law Stack Exchange
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Smithfield is one of the largest producers of pork/pig in the United States, but, apparently they are owned by a foreign (Chinese) company, would it be legal for US federal government to force sale of it to US company or US govt is USA govt thought Chinse government was using it for spying/etc?
No
Always assuming that the government has operated within the limits of its powers or, at least, that if they have exceeded those powers the excess was in good faith.
First, there is the issue of sovereign immunity. Basically, a government can be held liable only when it consents to be held liable. Most governments never waive this with respect to their lawmaking powers because they have to be able to make laws in what they see as the public interest without fear of litigation. See, for example, cases on plain paper packaging of cigarettes.
Second, most governments have the power to regulate commerce and to deal with public emergencies. There is an issue which comes up in Federations about which government has the power but even if a law is invalid, it does not follow that compensation is payable. It certainly isn’t if the law is valid.
This will depend on the details of the constitution. Generally, governments reserve sweeping powers for use in emergencies, often without automatic compensation.
In Germany, one of the EU countries you mentioned, the legal remedy for someone who feels unjustly singled out by a lockdown measure is to file a lawsuit against the measure, not to send a bill for the economic losses. At the same time, the government did decide to pay some compensation for some losses, but that was something between welfare and economic stimulus.