Short answers, pulled from the story.
"Get Back" was written by Paul McCartney and credited to the Lennon-McCartney partnership. McCartney developed the song from an offhand bass riff he played during rehearsals at Twickenham Studios on the 7th of January 1969.
Billy Preston was an old friend of the Beatles who joined the sessions on the 22nd of January 1969, playing Fender Rhodes electric piano. "Get Back" was the only Beatles single to credit another artist at the group's own request, released as "The Beatles with Billy Preston."
"Get Back" was released on the 11th of April 1969 by Apple Records. It debuted at number one on the UK singles chart on the 23rd of April, the first Beatles single ever to enter the chart at the top position, and it reached number one in more than a dozen countries including the United States, where it became their 17th Billboard number one, matching Elvis Presley's record.
The Beatles performed "Get Back" three times on the roof of Apple Studios on Savile Row, London, on the 30th of January 1969. Police arrived during the third performance in response to complaints from nearby office workers, briefly cutting the amplifiers, before the band finished the song. Lennon closed the concert with the remark, "I'd like to say thank you on behalf of the group and ourselves and I hope we've passed the audition."
The single version includes a chamber reverb effect and a coda with the "Get back Loretta" lyrics. The Let It Be album version, remixed by Phil Spector, adds studio chatter before the song, omits the coda, and appends Lennon's remarks from the rooftop concert. Both versions are built from the same master take recorded on the 27th of January 1969.
During Twickenham rehearsals, McCartney improvised temporary verses satirising the anti-immigrant views of MP Enoch Powell, whose Rivers of Blood speech had recently attracted widespread attention. The improvised lines referenced Puerto Ricans in the United States and Pakistanis in the United Kingdom. These verses never made the final recording and the song was rewritten before release.