Excel formula to look up for several specific words and if found save specific text to another column

Basically to get your desired output, instead of nesting each IF statement within each other, you need to write them separately and combine the output. You can use TextJoin to do that. Check the formula below:

=TEXTJOIN(", ",TRUE, 
IF(COUNTIF(A1,"*car*")>0,"Cars are big",""),
IF(COUNTIF(A1,"*flowers*")>0,"Flowers are beautiful",""),
IF(COUNTIF(A1,"*tree*")>0,"Trees are Green","")
)

Here each condition is evaluated separately and all of the results are joined into one text string. The TRUE in the formula denotes if you want to ignore empty values.

Below is an example of the output: