What is the extrusion multiplier called in OrcaSlicer?

Flow ratio. It sits in the filament profile, so each filament keeps its own value.

Are wall loops the same as perimeters?

Yes. OrcaSlicer calls them wall loops, PrusaSlicer calls them perimeters and Cura calls the setting Wall Line Count.

Not as one setting. You limit it through print speed, layer height and line width instead. The max volumetric speed calculator converts between them.

What it does	OrcaSlicer	PrusaSlicer	Cura
How much plastic to extrude	Flow ratio	Extrusion multiplier	Flow
Number of outer walls	Wall loops	Perimeters	Wall Line Count
How full the inside is	Sparse infill density	Fill density	Infill Density
Solid layers on top	Top shell layers	Top solid layers	Top Layers
Where each layer starts	Seam position	Seam position	Z Seam Alignment
Filament pulled back on travel	Length (under Retraction)	Retraction length	Retraction Distance
Nozzle lift on travel	Z-hop height	Lift height	Z Hop When Retracted
Shrink the first layer	Elephant foot compensation	Elephant foot compensation	Initial Layer Horizontal Expansion (a negative value)
Cap on melt rate	Max volumetric speed	Max volumetric speed	No single setting
First layer thickness	First layer height	First layer height	Initial Layer Height

Flow ratio and extrusion multiplier are both written as a number near 1, such as 0.98. Cura's Flow is a percentage, so the same setting reads 98%.

Cura's first layer expansion is a distance, and a negative value shrinks the first layer the way elephant foot compensation does in the other two.