Difference between Oil based Undercoat and acrylic undercoat

[Drew1000]

Was watching a you tube video by The Candleman which was very good in painting a new build property.  He used acrylic primer for door frames and oil based undercoat for skirting boards.  can anyone please explain the difference and any pros and cons and also where water based undercoats come into the equation (if they do).  Apologies for ignorance on paint but thats why Im here posting I guess.