WordPress post meta title can be prepared from different files based on installed theme and SEO plugin.
If you want to remove site name from post meta title tag, then there are 3 possible options.
Options are: Custom filter code in functions.php file, Specific plugin for customize post meta tag or SEO plugin which provide SEO title tag override option.
As per our analysis and testing, custom filter code needs extra care on theme or WordPress update.
And, good free SEO plugin is the best option to use for post SEO meta title change compared to specific post title update plugin.
For example, Yoast, Rank Math or other plugins.
To remove site name from post meta title tag, first install and activate Yoast or other SEO plugin, next open SEO title settings (for Yoast, “SEO > Search appearance > Content Types”), now remove site title and separator options from title from SEO title setting.
Hope you find this answer helpful.