I'm absolutely new to music theory and really wanna learn piano myself at home without teacher. I've got a new sheet for a song and trying to figure out why B flat major chord can have different notes. I attached an image. As far as I know, B flat major chord in two flats key signature is Bb - D - F. However as you can see below, F-Bb-Eb is also written as Bb in F position for inversion but it doesn't make sense to me. Thought other sus2,4 but not able to make it. I'm absolutely beginner about piano. so any kind help would be really appreciated.
The purpose of the chord symbols above the staff is to describe the harmonic structure of the music, not every melodic detail, as Bennyboy1973 notes in his answer. Think of the chord symbol as saying "if the guitarist (or second keyboard, or ukulele, or whoever) plays this chord, it will fit." Many pianists, if not most, will ignore chord symbols altogether.
In other words, you generally should not expect every note directly under a chord symbol to be a chord tone. Usually, most of the notes under a chord symbol will be chord tones, and that's the case here: the symbol is in effect for half a measure, and if you want to count note heads, 90% of them are chord tones. If you prefer to go by duration, the percentage is 92%.
It's not written as clearly as it could have been. The 'B♭/F' stands well for the rest of that bar, E♭ note resolves almost straight away to the M3 of B♭ - the D note.
All the other notes in the rest of that bar constitute B♭ major, so maybe the symbol could have been 'B♭sus4,' followed by 'B♭', but that takes up more space than there is.
Think of the chords as a bed or cradle and the melody weaves around it like a humming bird darting about flowers. When the melody plays these tension notes, which our brains thrive on because they resolve, you can think of them as passing tones or upper and lower neighbor notes. It is okay and desirable for the melody or chords to clash with one another. Again, tension. Our brains thrive on the resolution, like a sung Amen.
Every note doesn't have to be chord defined and you can just "throw them away." For example, play an A minor chord in your left hand (A C E) and in your right hand play this line: D B C F D D# E C B F#(lower). You can either see that as a melody with passing tones, upper and lower neighbors and tension notes or litter your page with melodic appropriate chords such as augmented, flat fifths, 11's, flat 13's, ninths etcetera.
If you want to be a teacher or composer, you need to know these things. If you want to be a performer, you need to feel them. Hone one or the other, preferably both. Guys like Oscar Peterson or Bill Evans love to target ninths, flat 5ths and flat 13ths. They both feel the tension and know which notes are needed to create those tensions. I don't think they think "I'm going to play a flat 13th here but rather, hear in their mind what they want to play and know where it is.