A new page inherits a head start when its site has already proven deep, trustworthy coverage of that exact topic, because Google extends the trust it has built in that subject area to fresh pages that fall inside the same lane. The mechanism is not a single sitewide boost applied evenly to everything you publish. It is topic-specific trust that follows the subject, so a new page lands on ground the site has already earned credibility on, while a page about an unrelated subject gets none of that lift.
What actually transfers is recognition. When a site has covered the core sub-questions of a topic, earned links to that body of work, and ranked for queries inside it, Google has accumulated evidence that this site handles this subject well. A new page on the same subject reads, to the crawler, as another contribution from a known and reliable source rather than an unproven stranger. That recognition shows up as observed behavior: the page tends to get crawled sooner, indexed more readily, and given a fairer initial position than the same page would receive on a site with no track record in the area.
The boundary is the important part. Authority extends within the lane it was earned, not across the whole domain. A site that is the recognized voice on home espresso machines does not pass that standing to a new page about tax law it suddenly publishes. The trust is bound to the topic, so launching outside your earned lane means starting cold again, even though the domain is the same.
This also explains why coverage compounds. Each strong page you add to a topic deepens the evidence that you handle it, which raises the floor for the next page on that topic, which makes the next one easier still. The advantage is cumulative inside the subject rather than borrowed from the domain at large.
When you plan your next pages, launch them inside a topic where your site has already earned its standing, so each new page inherits trust instead of building it from zero. Save expansion into a genuinely new subject for when you are ready to earn authority there from scratch, because the transfer will not carry you across that line.