My other question is, does every applicant have to attend an interview?
No, there are no interviews at all for UK visa applications made in the US. Biometrics are taken by US immigration on behalf of UKVI, so it only takes 5 minutes and they don't care what visa you are applying for, and then your application and documents are mailed to Sheffield for processing, so it's not physically possible for you to attend an interview anyway.
My basic timeline is:
1) Research the visa requirements and documents needed
2) Gather all the documents. Check and double-check you have everything to meet all of the requirements , making sure everything is in date and meets the formatting requirements
3) Print and fill out VAF4a Appendix 2 - this is the second half of the application form and contains all the questions relevant to applying for a spousal visa.
4) Start to complete the online application (which is generic information about you and doesn't actually ask any questions about how you qualify for the visa), but do not submit it until you are completely ready to mail all the documents to Sheffield for processing.
5) When ready to mail everything, submit the online application. You will be prompted to pay for the visa, pay the IHS surcharge and book a biometrics appointment at one of the 129 USCIS Application Support Centers across the US... DO NOT book at a premium centre as they cannot process your biometrics there unless you are purchasing the $1,700 Settlement Gold Premium Service (which is a waste of money anyway)
6) Print the online application and the biometrics confirmation sheet
7) Purchase settlement priority processing for $848 (if you are paying for priority) and shipping labels from VFS Global.
8 ) Print the priority receipt and shipping labels
9) Attend the biometrics appointment with US immigration (at the USCIS center), taking with you your passport and biometrics confirmation sheet. They will stamp the confirmation sheet, then take your fingerprints and photo and send you home... shouldn't take more than 5-10 minutes
10) Place your passport and stamped biometrics confirmation with your documents in the package
11) Place the return shipping label in the package, with your Priority Processing receipt on the very top
12) Write 'Settlement Priority Service' in very large letters on the outermost packaging
13) Mail documents to Sheffield for processing... this must be done within 5 days of biometrics
14) Wait for visa to be processed and mailed back to you in the US
15) Book your flight to the UK and move within the 30 days allowed on your visa vignette in your passport
16) Within 10 days of arriving in the UK, pick up your BRP card (your actual visa) from a designated Post Office