~ HOW TO INTERACT WITH A TOOT THAT HASN'T REACHED YOUR INSTANCE YET ~
(because it's a question I see a lot from new users)
1) Right-click on the timestamp to copy the toot's URL or open it in a new tab.
2) Paste the URL of the toot into the search bar in the Mastodon web UI, then press enter.
3) The toot will appear in the search results and you can boost or reply from there.