If you intend taking up permanent residence in the United States, you are required to wait until the immigrant or fiancé(e) visa is issued. You cannot reside in the U.S. on a tourist visa or visa free under the Visa Waiver Program while waiting the issuance of the visa.
However, if you wish to make a temporary visit at the end of which you will return to your permanent residence outside the United States, you may travel on a tourist B-2 visa, or visa free under the Visa Waiver Program, if qualified.
If applying for a B-2 visa, you will be required to furnish evidence of your residence outside the United States to which you intend returning at the end of your temporary stay. Although a pending immigrant or fiancé(e) visa application is not necessarily conclusive evidence of intent to abandon a U.K. residence, it is a factor considered by consular officers reviewing a visa application. If you are unable to convince the consular officer reviewing the application that you do not intend abandoning your residence, you will not be issued a visa.
Within the Visa Waiver Program, foreign citizens from certain countries have the right to travel to the United States if the purpose of the trip is a rest or a working visit. The duration of the visit should not exceed ninety (90) days.
If you plan to travel to another country and travel through the United States, in this case you will also need to obtain a travel or visa permit. If you plan to get an ESTA permission for this purpose, then, when answering the question “Do you transit through the USA to another country?”, You should definitely note the answer “Yes”.