By your question is it very clear that you are beginner in sequence analysis. So the easiest suggestion is download aliview software from the link below. And drap & drop +CTRL key - each fasta files separately. It will add ope fasta to another. And laste save as fasta. This will have your multifasta easily made. No need of any script or programm. You can even do alignment and several other tasks. This software is very handy in doing many bioinformatic tasks. And I almost use it daily.

https://ormbunkar.se/aliview/

Thank you so much! it works perfectly.

Hi Madeleine

I recommend seaveiw software. It can easily combine your data.