Products → Export → CSV. Your template now contains every column Shopify expects, with a real example row.
Column headers are sacred. Duplicate the example row and edit; never reorder or rename.
Image src must be a direct URL. Host via Shopify Files or any CDN; links must resolve at import time.
One product = one Handle; variant rows repeat the handle and set Option1/2/3 + price/SKU.
Import, open 3–4 random products, check price, variants and images. Fix at the CSV level, not product-by-product.