A lot of icon packs (if not all) are incompatible with One UI. So, I have a solution to bypass that, in order for you guys to enjoy those icon packs you had your eyes on, without the need to download a third-party launcher 😇.
1) Download Shortcut Maker.
2) Download an icon pack of your choice.
3) Then, go into the Shortcut Maker app, tap on apps, then tap on any icon. Then, select its icon variant, based on the icon pack you just downloaded and finally, create shortcut! Enjoy 😉.