301 redirect

In a 301 redirect, on for example old.com, a browser visiting old.com is instructed to request the new url = new.com  I am not sure whether any content on old.com is received by the browser but in any case any such content will not be loaded in the browser window so, in terms of being loaded in the browser at least, the content on old.com is ignored or ineffective.

Can anyone please tell me what happens to the content on old.com eg how far it gets in the sequence of the original browser request?

Anyway I am trying to understand the Apple iTunes instructions re changing a feed url here
https://www.apple.com/uk/itunes/podcasts/specs.html#changing
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If possible, you should do two things:
1) You should use the <itunes:new-feed-url> tag...
2) You should set your web server to return an HTTP 301 response and redirect
when receiving a request for the old feed.
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So the instructions seem to say add a <itunes:new-feed-url> tag to the old url and also put a 301 redirect on the old URL.

I thought that a 301 redirect meant that any content on the old url (such as the <itunes:new-feed-url> tag) would possibly or probably be ignored. Can the <itunes:new-feed-url> tag on the old feed URL be effective when there is a 301 redirect on the same URL?

Thanks...