"If multimeters can only handle low current (A) loads for DC, why it can handle much larger loads for AC ?"

AC voltage is peak to peak or .707 * DC voltage.

"What are the proper terms to describe the opposition to a DC current and an AC current? Do they use the same unit?"

DC "opposition" unit of measurement is the ohm. AC's is frequency dependent and is also measured in ohms, but it is referred to as the circuit's impedance.

__________________
Chris
1985 300SD - 'Grace' (198K mi.)
2018 Honda Civic Sport
2018 Honda CRV LX
2010 Honda Fit Sport (RIP)
2013 Honda Accord Sport (Sold)
1996 Lexus LS400 (Retired)
1995 Ford Contour SE (Retired)
1976 Porsche 914 (Sold)
1972 Datsun 240Z (RIP)