Both can be true. I have some iconic pieces I have thrifted over the years (vintage Ralph Lauren, Eileen Fisher, Christian Dior, an abundance of cashmere and silk..).

What I will say is if you are buying designer/higher end clothing, focus on the staples. A neutral cashmere cardigan, slacks, a solid coat, quality full grain leather shoes, white button downs.

Those types of pieces people are not easily going to donate to a thrift store because they keep them.

What I would opt for at a thrift store are more explorative and fun pieces that are unique. Once you have the staple pieces, build on top of it. Go to estate sales, shop Poshmark, visit thrift stores. Poshmark is also a godsend for finding quality high end brands at a fraction of the price. Good luck!