An emulsion is just a stable mix of two things that would normally separate, like oil and lemon juice. Once you understand that, mayonnaise, hollandaise and vinaigrette stop feeling like three different recipes and start feeling like one technique with three variations.
The core idea is to add the oil slowly to a base while whisking vigorously. The base is what determines the emulsion: egg yolk and lemon for mayonnaise, egg yolk and reduced lemon for hollandaise, mustard and vinegar for vinaigrette. Start with the base, drizzle in oil drop by drop at the beginning, then in a slow stream once it thickens. If it breaks, do not panic: start a new base in a clean bowl and slowly whisk the broken sauce back in.
For hollandaise specifically, the second variable is heat. Whisk yolks and lemon over a double boiler until they thicken to ribbons before adding the warm clarified butter. The butter should be barely warm, not hot, or it will scramble the yolks. Once you have made the same emulsion a few times, you can make any one of these in 5 minutes flat with a whisk and a bowl.