Amos and the others were in Martian armour, just not the full on recon armour Bobby has which is probably in scarce supply. So they would have had some protection, just not the full body armour Bobby has. Amos doe get shot while protecting her but as long as the hit you take isn't instantly fatal the suits are able to seal wounds and pump your body with drugs to keep you going until you can get it taken care of (from the books). I think in his mind he was just going to sacrifice himself to protect her as her suit was about to fail. Then the Roci turns up.
Plus main characters have plate mail +4 while extras get the regular stuff.
Aye Bobby was basically Martian Marine ground assault, from memory a fairly elite part of the Martian armed forces who amongst other things were trained to operate in higher than 1g with ease (remembering that most Martians were not physically used to more than .6g), and her armour is the full up assault/heavy power armour needed to carry the full weapons outfit and amongst other things allow them to do long range/longer duration missions and jobs that include front line combat and assaulting fortified locations.
The rest of the team were wearing what is basically light armour, primarily intended for shipboard security/general use, with protection against light fire., but it's made of lighter/cheaper materials and misses any of the power assist functions.
You can see the difference to a degree when they're about to board the observation ship earlier in the series, everyone else is knocked over/struggling to stand in their standard Martian armoured suits, whilst Bobby is able to stand and move fairly easily as her armour kicks in.
IIRC all the Martian ships carry multiple sets of the light armour as standard (they're the suits you see on the Rocinante hanging up), but the Marine armour goes with it's user because it is customised and too expensive/limited/sensitive to have it sitting around away from trained personal.
Basically Bobby was wearing a light tank in terms of armour, whilst everyone else was wearing kevlar with plate carriers.
And as you say all the suits, right up from the belters basic ones have some ability to seal around punctures* to prevent people dying due to minor accidents, I can't remember if in the books the suits are described as having the ability to apply medications to help reduce shock etc or not (it's a fairly common thing in even basic suits in many sci fi books, along with things like automatic tourniquets on more advanced suits so major damage to a limb will result in the suit basically writing it off to save the wearer).
*We already see similar tech in use in aircraft fuel tanks (especially combat aircraft), and the goo you put in tyres either to fix a puncture or as a preventative if it's likely to be exposed to hard use.