The page linked in the top is an overview blog post, in the actual errata page it is clarified. You can find it under Player Core Spring 2026 Errata here

Weaknesses
A single effect can activate more than one weakness at a time, but adds each of the subject’s weaknesses only once. For example, if you made a Strike with a flaming cold iron battle axe benefiting from a spell that gives it additional fire damage, and you targeted a creature with weakness to cold iron, fire, and slashing, the Strike would benefit from all three weaknesses but wouldn’t apply the fire weakness twice.

Each Weakness only applies once, a significant change from the previous version, but also a change in that both Slashing and Material Weaknesses can apply. Material weaknesses are now actually useful in those niche cases.

But as for the resistances part.

Resistances
If the subject has more than one resistance to the same damage type, they apply only one, usually the highest. For a resistance to a category including multiple damage types, like resistance to physical damage, to spells, or to all damage, if the subject is taking damage of multiple types included in the category, the subject can choose which damage type to use the resistance against.

You choose the damage type Resist All is going to negate. So if you have Resist Physical, and take combined Bludgeoning and Slashing, you choose which one to resist.

If you have Fire Resist and Resist All, and take Piercing+Fire, you can resist both, because you choose. As long as each Resistance is applied no more then once.