Instagram feed preview
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Shows your image cropped to the square your profile grid uses, which is not the shape you posted; a 4:5 portrait loses its top and bottom on the grid.
How to use the instagram feed preview
The grid crop is the thing people plan for least and notice most. Your feed shows a 4:5 portrait in full, but your profile grid crops the same post to a square from the middle, cutting roughly 12.5 percent off the top and the bottom. Text near either edge survives in the feed and disappears on the profile. If your grid is part of how the account reads, and for a brand or a portfolio it usually is; compose so the important content sits inside the central square and treat the extra portrait height as breathing room rather than as content.
Questions
Yes, to a square, taken from the middle. A 4:5 portrait loses about 12.5 percent top and bottom.
Instagram lets you adjust the profile grid crop on newer versions, but the default is centred.
Only if the grid matters more than the feed. Portrait uses more feed height, which is usually worth more.
It shows one tile. Judging a whole grid needs the app or a planning tool.
No, and it cannot be. Platforms cut by rendered pixel width, so a title in capitals or wide letters cuts earlier than a character count suggests. Treat the limit as a guide and leave a few characters of margin.