Which half star to use
Unicode has four true half stars, all added in version 11.0 in 2018. ⯪ STAR WITH LEFT HALF BLACK and ⯫ STAR WITH RIGHT HALF BLACK are full star outlines with one side filled, which is exactly what review sites show for half a point. ⯨ LEFT HALF BLACK STAR and ⯩ RIGHT HALF BLACK STAR are only the filled half, with nothing drawn on the other side.
For ratings, use ⯪: put it after your filled stars ★ and before your empty stars ☆, as in ★★★⯪☆. ⯫ is its mirror image, useful for decoration or right-to-left layouts.
The ✬ fallback
✬ BLACK CENTRE WHITE STAR is a dingbat from 1993: an outlined star with a solid middle. It is not officially a half star, but it reads as "partly filled" and shows up on almost every device, so many people use it when ⯪ appears as an empty box. ✫ (open centre) and ✭ (outlined black star) are similar looks.
Because ⯪ ⯫ ⯨ ⯩ are fairly new, older phones and some apps have no font for them. Check how your rating looks on the devices your readers use, and fall back to ✬ if needed. There is no half star emoji.
Writing star ratings everyone understands
Keep ratings to five symbols in a row, use a single font, and add the number as text, for example "★★★★⯪ 4.5/5". Screen readers read each symbol by name, so the number also helps blind and low-vision readers.
Need plain stars instead? The star symbol page has every filled and outline star, and the circle symbols page has half-filled circles like ◐ and ◑ for progress and moon phases.