In Mono mode, sample playback and envelopes are always retriggered regardless if you play staccato, or in a legato style—play a new key while holding another.
In Poly mode, you can play several notes simultaneously.
Note: In many classic monophonic analog synthesizers, the behavior in Legato mode is referred to as single trigger, while Mono mode is referred to as multi trigger.
In the Synth Details slide-out pane of Logic Pro Sampler:
Choose Mono or Legato from the Mode pop-up menu, depending on the keyboard mode you want to use.
Set the number of unison voices with the Unison pop-up menu.
The intensity of detuning—voice deviation—depends on the value you set with the Random Detune field. See Apple Books | PDF