i would like to 301 the entire old domain to the single new domain page for seo.
...
what do you think?

this 5 year old thread is still informative on this subject:
Google seeing 301 Redirects as Soft 404 Errors [webmasterworld.com]

since i am not a tech user to fully understand that.
Can you please exaplain in short?
Thanks

Redirecting old pages to a single page, even if you use a 301 (permanent) redirect does not redirect visitors to anything close to what they expect to see when they use the old link. That is called a "Soft 404" because the old page is gone. It does not exist and was not replaced with a new page but rather than supplying a 404 (Not Found) error as you should or a 410 (Gone) message, you are sending people to a page (or site) that they did not ask for. Google sees that as worse that letting them get a 404 error.

It is the same as visiting a restaurant and ordering fried chicken, but they are all out of chicken, so you get an octopus salad instead. You probably would not like that kind of service.

actually it would be more like everybody who ordered anything on the menu at the old restaurant would get sent to the new restaurant and then all would be served the octopus salad instead.