Let me start by saying all of these points, in my opinion, is something we’ve covered here before but the interesting thing is that Google felt these seven items are things SEOs who give presentations have confused over the past several months. Thus, Google has issued a clarification around how mobile-first indexing works and what it really means for webmasters.
Here is the seven point list really short:
(1) URLs: Google will show the mobile URL to those searching on mobile search and the desktop URL to those searching on desktop, if there are separate URLs. But Google will still only index it as a mobile device would see it.

URLs in search: With Mobile-first indexing, we index the mobile version. When we recognize separate mobile URLs, we’ll show the mobile URL to mobile users, and the desktop URL to desktop users – the indexed content will be the mobile version in both cases.
— Google Webmasters (@googlewmc) June 14, 2018
(2) Crawl changed: The crawl rate and such should not change, although if you look carefully, you will probably see more crawling on your mobile pages over your desktop pages. There may be a temporary spike in crawl if Google needs to reindex everything.

Crawled counts: The total number of crawled URLs/day generally won’t change, but the balance will shift from mostly-desktop to mostly-mobile crawls. During a switch-over to mobile-first indexing we may temporarily crawl more as we reindex everything.
— Google Webmasters (@googlewmc) June 14, 2018
(3) Cache bug: As we covered recently, there is a bug where the Google cache will 404 on pages that were recently moved to the mobile-first indexing process. It is a bug with the user interface and has no impact on crawling, indexing or ranking.

Cached page: Unfortunately, it looks like we’re currently still not showing a cached page for many mobile-first indexed sites. This is a bug, not by design, and should get resolved over time. It’s just the UI, it doesn’t affect crawling, indexing, or ranking.
— Google Webmasters (@googlewmc) June 14, 2018
(4) Speed Update: Again, we covered this before, but the Speed Update coming next month is unrelated to the mobile-first indexing changes.

Speed and mobile-first indexing: The mobile speed update in July is independent of mobile-first indexing. Fast sites are awesome for users, especially on mobile, since devices & connections there tend to be slower than with desktops.
— Google Webmasters (@googlewmc) June 14, 2018
(5) Hidden/tabbed UI content: Again, covered numerous times, content in hamburger menus, accordions, tabs, expandable menus, etc on mobile work fine.

Mobile website UIs: Using “hamburger-menus” and “accordions” on mobile websites is fine.
— Google Webmasters (@googlewmc) June 14, 2018
(6) Desktop sites: Desktop sites, non-mobile friendly web sites, mobile sites, responsive sites, all sites will eventually move to the mobile-first indexing process. It doesn’t matter if you have a desktop only site, you will still be moved. In fact, we saw desktop only the first to move to mobile-first indexing.

On requirements: Neither mobile-friendliness nor a mobile-responsive layout are requirements for mobile-first indexing. Pages without mobile versions still work on mobile, and are usable for indexing. That said, it’s about time to move from desktop-only and embrace mobile 🙂
— Google Webmasters (@googlewmc) June 14, 2018
(7) Ranking Boost: Yes, Google has a mobile-friendly ranking boost and that will stay, it has not changed. Going into mobile-first indexing does not change that fact. There is no additional ranking boost for being mobile-first indexing – there is for being mobile-friendly.

On ranking: The mobile-first index doesn’t change anything for ranking other than that the mobile content is used. While mobile-friendliness is a ranking factor on mobile, being in the mobile-first index is not.
— Google Webmasters (@googlewmc) June 14, 2018
Again, non of this is new but Google felt the need to state these things again.
One thing Google should have added, as Pedro Dias mentioned would be number 8, that it is one index, the process of indexing is different, but it is a single index, not a separate mobile and desktop index:

And remember: *It’s the same index* https://t.co/tcO6s1KqAP
— Pedro Dias (@pedrodias) June 14, 2018
Forum discussion at Twitter.

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