if a friend hires someone to fix something in their house, but after
the repairs are done, the friend refuses to pay, this would likely
fall under theft of service.
This is incorrect.
The most common distinction is intent. (The federal Canadian criminal code provision for theft (Section 332) requires intent to commit the crime, although "false pretenses" (Section 362) is probably a better match within the Canadian criminal code to theft of services and also requires a showing of intent).
If breaching a promise to pay that the person making the promise to pay intended to be honor when the promise was made fails to pay (e.g. due to poor budgeting or losing a job) this is a breach of contract.
Similarly, if someone accidentally attaches the wrong cable line to their TV and gets the premium package rather than the regular one, but didn't realize that fact, the cable company might have a claim for negligence or for unjust enrichment, but this would not constitute theft.
In contrast, obtaining services knowing at the outset that you do not intend to honor your promise yet deceiving the service provider with your promise is theft. Theft (in the context of a theft of services) is intentionally obtaining services by deceit with an intent not to pay for them at the time that they are obtained.
Proving this intent is usually prohibitively difficult and no something that law enforcement will choose to press charges regarding, but with very clear evidence (e.g. an audiotape of the person making a promise to pay bragging immediately before or after making the promise to pay about how he never intends to pay in the first place, or in a case with a pattern of conduct involving many service providers on multiple occasions), charges can be pressed and a conviction can be won.
Also, to be clear, it has nothing to do, per se, with the power relationships of the parties. If an employer picks up a bunch of day laborers telling them that they will be paid an agreed rate at the end of the day for the work they are made to do, and the employer not only doesn't pay them at the end of the day but didn't intend to do so in the first place and perhaps has a practice of picking up day laborers and then refusing to pay them without good cause, that is theft of services a.k.a. wage theft.
if I play a movie from a streaming service and screen-share on Zoom, I
would be committing civil copyright infringement (if it is not fair
use), but if I give other people my login credentials for the
streaming service, it would be theft of service, I think, depending on
the jurisdiction
This isn't a good analogy as it implicates copyright law which is quite different from other bodies of law.
Some acts are both civil and criminal copyright violations. But criminal copyright violations are generally not a theft of services.
In Canada, the typical criminal copyright violation involves the sale or rental of the copyright protected material to third parties for money.
how does this distinction affect the outcome for a person
liable/guilty of either?
Civil law violations are a basis to bring a lawsuit against someone who breaches the law in a civil manner for money damages sufficient to compensate the person bringing the lawsuit for the damages that they have suffered.
Theft is a criminal offense. The prosecutor's office decides whether or not to bring charges and if it does bring charges does so (in Canada) in the name of the Crown, at the government's expense under criminal procedure rather than the civil procedure applicable to lawsuits. The victim is usually a witness and is usually consulted, but is not a party to a criminal case. If a conviction is obtained for a criminal offense, the penalty is usually some combination of incarceration, a fine, probation and community service, as authorized by statute, with court costs and restitution to the victim tacked on as an afterthought. The measure of what is owed as restitution is typically more grudging than the measure of the damages that can be awarded in a civil lawsuit.
The two remedies are not mutually exclusive. Someone who is prosecuted for committing a crime can also be sued if the evidence supports both civil and criminal claims. Someone who is sued can also be prosecuted if the evidence supports both civil and criminal claims.