When the Internet started, it was a U.S. defense project funded by an agency called DARPA. It was not supposed to be used for commercial purposes. However, commercial contractors were allowed on the network to facilitate communication e.g. between defense agencies and contractors. The domain names were used to identify who was who: .mil for military, .edu for universities, .org for nonprofit organizations, and .com for commercial enterprises. Originally, it was OK to send email from, say, .edu to .com or .edu to .org or .org to .com... but not directly from .com to .com. Since the Internet originated in the United States, there was no need to have a .us domain name--it was all U.S. So, historically and typically, organizations in the U.S. have .edu, .org, or .com domain names rather than .us domain names.

.com predated the .country TLDs(Top Level Domains). I think that originally the idea was to have TLD based on function, rather than country. When country TLDs became common for countries other than the USA, American companies did not generally adopt the .US TLD. In general, you can't make any assumption from someone's TLD where they are from. One particular example of that is .tv - although it represents Tuvalu, Tuvalu makes significant income by selling .tv domain names. It is very unlikely that anyone with a .tv domain name would be located in Tuvalu!

Look up the company on Wikipedia. It will tell where the company originated from.

Dot com means company/business.