When the rout in technology heavyweights like Facebook Inc., Amazon.com Inc. and Alphabet Inc. spilled over into the broader market in late March, many feared that individual investors would flee for the exits, exacerbating the declines.

Instead, Alex Boucher, a 22-year-old college student in Westfield, Mass., added to his position in chip maker Nvidia Corp., seeing a chance to pick up the pricey stock at a discount.

San...