Per the PHB, p. 195, as quoted by you:
Most creatures have a 5-foot reach and can thus attack targets within
5 feet of them [...].
RAI, PCs have a 5-foot reach around them
Since the PHB explains the core rules for players to follow in game, it is safe to assume that "most creatures" includes Playing Characters, as creatures of Medium or Small size; realistically speaking, a regular-sized humanoid can somehow interact with things within 5 feet from them without moving, especially with a weapon or a tool.
RAW, there is no specific mentioning of a character's reach
That is the only paragraph where reach by itself is explained in the core rules and it's paired with the concept of attacking. It is never included in a creature's characteristics, like Size or Speed, neither in the DMG or in the MM.
Indeed, reach is regularly mentioned in the description of attacks, e.g., a Wolf's Bite:
Melee Weapon Attack: +4 to hit, reach 5 ft., one target
There's a correlated concept mentioned in the Space section of PHB (chapter 9, p. 191, emphasis mine), about the "cells" occupied by a creature:
A creature's space is the area in feet that it effectively controls in
combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a
space that wide.
Nothing else is said about what is around a creature's occupied space and their influence on said surroundings.
This means that there are no detailed rules about creatures' reaches for interactions that are not attacks and the paragraph you quoted is the only generic guideline.
Since no specific ruling is given, the DM decides
The DM has to decide what can and cannot be done.
When not attacking, does a Huge creature have, say, a 10-foot reach, thanks to their size, or just the "regular" 5-foot reach? Maybe that's the case for a Giant, thanks to their long arms. What about a Gargantuan one, does it have an even bigger reach? What about the Kraken, whose Tentacle attack has a 30-foot reach? Does that mean it can interact (without attacking) with things within 30 feet of it, thanks to its tentacles? Can a Dragon use their long tail to somehow interact with something out of the reach of their Claws or Bite?
If the DM rules that large creatures have a bigger reach thanks to their size, you could take advantage of this by Polymorphing, Shapechanging, etc., into a Huge creature, for example.
Regarding items, weapons with Reach, such as the Pike, add 5 feet to your regular reach when attacking with them (PHB p. 147, emphasis mine):
Reach. This weapon adds 5 feet to your reach when
you attack with it.
However, you might be allowed to creatively use them for something other than attacking, in a plausible way. Referring to the examples you gave in your question body, helping someone getting out of a Net might require you to directly touch it, or somehow cut it from afar with your Halberd; you could help someone getting out of quicksand by reaching out to them with your Lance, increasing your reach beyond your arms.
Similarly, Bugbears, as a playable race in Volo's Guide, have a racial trait that grants a longer reach when attacking. Your DM might allow this to be extended to maneuvers other than attacking, if you play as a bugbear or are transformed into one.
Long Limbed. When you make a melee attack on your turn, your reach for it is 5 feet greater than normal.
Unfortunately, there is no spell that directly increases a creature's reach, in the PHB. As previously suggested, spells which increase size might indirectly increase reach as well, if the DM rules so.