It looks strongly clivia-like to me. But Clivia is not a bulb.

thank you. but it is a bulb... I thought of amaryllis, but I think it's something

or maybe crinum?

I don't know any crinums that look like hippeastrums.

as I said, I don't know at all what this

Where did you buy them? Nursery... grocery... side of the road?

What does the bulb look like?

side of the road. the bulb for me looked like a large daffodil one but not so big

It does look very much like hippeastrum but I have no idea what kinds if any are grown in Vietnam. Based on how well it's growing maybe it won't be too long before a bloom shows, although you may have to work out when it needs to go dormant.

finally a bud! what is that?

Looks like an Amaryllis relative from the bud. Do post a picture of the open flower!

I will! haha I think it will be the regular red

Whatever you're doing keep doing it, that looks very promising!

what is that?! not the regular amaryllis, since it's narrow. can't wait anymore

Looks normal to me. Undersized bulbs don't always have a full complement of trumpets.

but what variety do you think it

and finally! it opened. different from the ones which I've seen before. thanks for following it with me

It's a South American hippeastrum, my grandmother had a pot full of this fifty years ago. I believe it's one of the species, but a precise identification has never been made (by me).
There's a close resemblance to three species from here.

https://www.pacificbulbsociety.org/pbswiki/index.php/HippeastrumSpeciesOne

which three species? I'm really confused between hippeastrums

thank you very much! I don't think it's an hybrid, but thank you