"My products aren't in any collections after import" — CSV doesn't assign collections

There's no Collection column to fill in

This is a genuine gap in the standard product CSV format, not a mistake on your end. Shopify's product import recognizes a fixed set of headers — Handle, Title, Vendor, Tags, Variant Price, Image Src, and so on — and Collection isn't one of them. If you added a column called "Collection" yourself hoping it would work like Tags or Vendor, the import simply doesn't recognize that header and ignores the column entirely, without any warning that it did.

How collections actually get assigned

Shopify has two mechanisms for putting a product into a collection, and neither runs during the product CSV upload. A smart (automated) collection uses a rule you set up once — "product tag is equal to summer-sale," for example — and Shopify automatically pulls in any product, imported or not, that matches. A manual collection requires adding products to it explicitly, either by hand in the admin or through a separate import made specifically for collection membership rather than the product catalog itself.

How to actually get this working

If your goal is to have newly imported products land in the right collection automatically, put a distinguishing value in the Tags, Product Type, or Vendor column during the product import — those columns do exist and do get set — then create or edit a smart collection with a rule matching that value. For a one-off batch assignment instead, use Shopify's separate collections import (Products → Collections) once the products already exist.