You can run your fulfillment through Amazon’s Fulfillment by Amazon (FBA)
program, in which Amazon handles all the storing, packing, and shipping of your
products. Here are some resourses explaining the FBA procedure in detail:

https://sell.amazon.com/learn/ecommerce-fulfillment

https://www.junglescout.com/lp-htsoa-guide

https://www.bigcommerce.com/articles/omnichannel-retail/amazon-fba/

In the same way, Walmart also offers Fulfillment services like Amazon. Information about this is below.

https://marketplace.walmart.com/walmart-fulfillment-services

Please check  the comparison:

https://www.ecomcrew.com/walmart-wfs-vs-amazon-fba/