which is the best command to use

[lina <lina.lastname@gmail.com>]

Hi,

(How are you all?)

well, I have two files:

File_a.txt
a
a
a

File_b.txt
b
b
b

I wish to get a file_ab.txt as
a b
a b
a b

I tried, but still don't know which is the best way to do it,

Thanks ahead and wish all have a nice holiday,

Best regards,