After officially becoming the Democratic party's candidate for president.Joe Biden took the political fight to his republican rival president Donald Trump, focusing criticism on what he said is the president's inablity to contain the coronavirus pandemic in the US.

1. 划线部分为现在分词短语作状语。

2.  focus A on B：把 A 集中于B。即： 把批评的焦点放在what 从句表示的内容上。

这里，on 表示“集中于；对准”，类似 fix one's eyes on sth. 的用法。

3.  on 的宾语为 what 引导的名词性从句，中间 he said 为嵌入成分。因此，这个 what 从句属于“嵌入式名词性关系分句”。又如：

Say what you think is most urgent.

拣要紧的说。

《新英汉大辞典》

Just do what you think is right.

《牛津词典》

This is what he thinks is important.

what is important 是 he thinks的宾语从句。what = the thing which,  所以本句等于：

This is the thing which he thinks is important.  （变成嵌入式定语从句）