How to typeset a chord with variable note length in Lilypond?

Question

I am typesetting a score using Lilypond but I am struggling with the following bar:

I tried <fa la> <<fa4 {la8 sib}>> … but it does not render properly.

Global settings.

\version "2.20.0"
\language "italiano"

Staff settings.

\key fa\major
\clef bass

Answer 1

While Glorfindel's answer is correct, it's a bit long-winded. Depending on your needs and the rest of the score, you can get the exact same results by simply inputting

<<
    {
      la4 la8 sib
    }
    \\
    {
      fa4 fa
    }
>>

In other words, you can do this without all of the \new Voice = "this" and \voiceThat commands.

Answer 2

It looks like you could use some single staff polyphony.

The following should work; it sets up two voices, one with stems up, the other with stems down, so they can play/sing different rhythms.

\score {
    \new Staff <<
        \key fa\major
        \clef bass
        \new Voice = "first" { \voiceOne la4 la8 sib }
        \new Voice = "second" { \voiceTwo fa4 fa }
    >>
}

Output:

Answer 3

Modifying @Richard's answer.
Putting \relative do at the start of each voice keeps the voice in the same octave (in the second voice).
Using q to repeat chords is very useful.

<<
   \relative do' {
    la4 la8 sib do4 do8 sib |
   }
   \\
   \relative do {
    fa4 fa <fa la> q |
   }
>>

Answer 4

This was a little bit confusing to me so I'll post a full working example (kudos to @elements in space for the hint) as I had issues understanding basic sheet concepts

\version "2.24.1"
\language "espanol"
\score {
    \new PianoStaff <<
      \new Staff = "first" \relative {
        \key re \major
        re''8[mi]
        la[sol]
        fas4
      }
      \new Staff = "second" \relative {
        \key re \major
        \clef bass
        <la dos>4
        << { dos } \\ {la8[mi]} >>
        <fas dos>4
      }
    >>
}