How do an ability-setting item and a +1 feat interact?

I’m playing a Half-Elf Lore Bard at level 5, with CON score of 14 after racial bonus.

Yesterday evening, a great oracle gave my character an Amulet of Health, which does the following:

Your Constitution score is 19 while you wear this amulet. It has no effect on you if your Constitution is already 19 or higher without it.

I’m quite happy as my character gain more HP, and my concentration checks will be improved.

In my initial plan, I wanted to take a feat at level 8: resilient (CON) which gives my character +1 CON. I still do, to at least gain the proficiency in Constitution.

If I take that feat, what will be my character’s Constitution score? 19 or 20? Basically, how do a set ability score and a modifier interact?

When planning on D&D Beyond, it says my CON is 19, but is it correct? Why? Which rules are involved here?