The VIC-20 memory map is divided into eight blocks of 8K each. Five of those are available on the cartridge bus (well, actually seven but the last two are I/O extention blocks). Each cartridge can map one or more blocks, usually only two at a time.
It means you could plug in e.g. a 16K memory expansion allocating blocks 1+2, a Super Expander allocating half of block 5 and a Programmers' Aid allocating half of block 3. Or maybe a Vicmon machine code monitor allocating the second half of .. well, I can't remember if it is 3 or 5. Or a game cartridge in block 5, but it would be auto starting anyway.
If you plug in two cartridges allocating the same memory block, I think the first one gets preference, but I'm not entirely sure how it works electronically. Some other expansion boards had switches or buttons which would let you choose one of many cartridges, but at the expense you could not have multiple connected simultaneously as I described above. Which approach is better or worse depends on what kind of user you are.