Some of my best clothes were thrifted. I shop a lot of places, but the highest quality clothing I've thrifted mostly came from private church thrifts or Junior League thrifts. Don't overlook consignment second hand type shops if you are shopping specifically for a professional wardrobe.

Learning about fabric is a biggie, and I have no shortcuts to offer, all require time & touching the good stuff. I've sewn since I was a kid, so spending time with fabric is something I grew up doing.

You can go to a high end department store & look through the clothes, particularly the business wear. Feel the jackets, pants & skirts. Good suits for both men & women have a minimal amount of manmade fiber. Touching a pair of jackets where one is 100% wool and the other 60%/40% will give you a good lesson that no internet or book description can match. Nice infographic with lots of names you might see on labels: https://sewguide.com/textile-fibers/

Know which brands actually last vs. are trendy and popular. How does this shirt you know was popular two years ago look in the thrift store? that low end fast fashion stuff will teach you just as much because you can see what to avoid. Find two similar items & compare them. Look at the seams, look at fiber content, look at what wore out/ tore/ stretched badly.

Personally I look for Ralph Lauren for both me & my husband, Geoffrey Beene for him, Talbots for me. There are other good brands, but these fall into our specific 'will always fit' (hubs is a XXLT) or 'will always suit my style & work dress needs'.

I wish you luck!