Attacks require an attack action
Attack is defined on page 12 Core Rules:
When a creature tries to harm another creature, it makes a Strike or uses some other attack action. Most attacks are Strikes made with a weapon, but a character might Strike with their fist, grapple or shove with their hands, or attack with a spell.
The actions described here would be those that use an attack action, likely the same that have the attack trait, not just anything that can damage you. A fireball, for example would not count, because it is not caused by an attack action.
Physical in relation to attack is not a defined game term. It likely means corporeal or bodily, as opposed to incorporeal, because the term is used in this context repeatedly, for example in the Physical Training feat, in the Manipulate trait, or, especially with incorporeal creatures, where the defintion states:
An incorporeal creature or object has no physical form. It can pass through solid objects, including walls.
So any attack that cannot pass through a solid object would count as physical, and would be blocked by your Shield Block.
Regarding the four examples you list:
- Critical Trip: I think this is somewhat of a DM call. It is an attack, and it does cause damage, but it is not the attack that causes the damage directly, it is the resulting fall to the ground that is espcecially hard. The Shield does not prevent you from falling (because it only does prevent damage, and a normal trip does not cause damage, it just sends you prone), and is not warding off a blow here. I personally would rule that you would take the fall damage, but I can see another DM going with a more technical reading. The Text of Shield Block says:
You snap your shield in place to ward off a blow. Your shield prevents you from taking an amount of damage up to the shield’s Hardness. You and the shield each take any remaining damage, possibly breaking or destroying the shield.
Telekinetic Projectile. This is an attack, and it is hurling a loose, unattended object, clearly a physical attack that can be blocked by an interposed shield. Shield Block works.
Scatter Scree is not an attack. Shield Block does not protect against it. It also does not make sense narratively, this is an area effect with "a jumble of rocks in the area", it is not a single blow, and there would be rocks hitting you from all sides, that you can not all block at the same time with interposing the shield.
Cinder Rat strike this is both an attack, and, even though made from fire, physical in the sense that it cannot pass through a solid object, like a wall or a shield. If you use that definition of physical, Shield Block will protect against it. If you instead believe a physical attack needs to deal physical damage, this one would not be blocked. As it is not spelled out which of the two is intended, this also leaves some room for a DM call.