Redirect products on one domain to product page on shopify on a different domain

[DeeLT]

We currently have two websites/domains (don't ask).  Years ago the parent company required us to set it up like this but we don't need it like that now so we're merging the two websites into one shopify site.  

Domain A is currently a wordpress website with no ecommerce.  This is the domain we are keeping and going to be using on our new Shopify site.  I have listed all the current pages and their new url in a spreadsheet that I will upload to shopify.  These redirects are easy and I have this part covered.

Domain B was our Bigcommerce site and we are moving the e-commerce to shopify on Domain A.  I need to know how to setup redirects for each of the products on this site/domain so that when someone goes to domainB/bibb-rex they are redirected to domainA/products/lettuce-bibb.  I need to do this for all 100+ products, for obvious SEO reasons.

What is the best way to set this up? I've reached out to Shopify support, but they've not been much help.  I'm getting silly AI answers that doesn't address one url/domain to another alongside the other domain.  Then they said create an A record, but I'll have an A record for the domain I'm keeping that will be live.  I plan to renew DomainB for at least a year, maybe longer.

[DeeLT]

livingtowers.com is the main domain we will use going forward. There are
blog posts and pages on this domain and I'm trying to retain their SEO
livingtowerseedlings.com is our current bigcommerce site. I need these
products redirected to their new product on the new shopify site that will
be at livingtowers.com.

I have the complete list of all urls from and to in a csv file.

[SEOAnt-Jeffery]

Hi @DeeLT

Suggest you consider making redirections from domain B to domain A gradually. After that, put all your efforts on domainA and set it as the main domain on Shopify, so that no matter which URLs are accessed by your visitors, you will not encounter 404 errors. So your conversions or traffic will not get affected too much.