Breeding is incredibly useful for creating high IV Pokémon, but in Generation 8, it's not the end all, be all. After completing the game, you'll be able to enhance your Pokémon's IVs and change their Nature at the Battle Tower in Wyndon.
How to Use
To use Hyper Training, speak to the right-most fellow at the back counter. He'll let you Hyper Train any Pokémon that are Level 100. You can then give him a Bottle Cap for each stat that you want to have max IVs, or a Gold Bottle Cap to max all 6 stats at once.
When you've first completed the game, your Pokémon will probably be around the Level 60 range--far from the Level 100 goal. The quickest way to Level Up is to engage in high difficulty Max Raid Battles and feed your Pokémon experience candies obtained there. This can be done online or (slightly harder) offline.
Bottle Caps (and gold ones) can be obtained randomly from the Digging Duo or you can buy the regular ones for 25 BP from the clerk who's second from the left. You can earn BP by battling in the Battle Tower or online battles/competitions. You can also earn both kinds of Bottle Caps by reaching certain milestones in the Battle Tower.
Mints can be purchased from the left-most lady at the back counter, for 50 BP apiece. You can also obtain Mints by reaching certain milestones in the Battle Tower. Once you've got a Mint, you can feed it to a Pokémon directly to change its boosted/lowered stats.
A Cosmetic Change
Hyper Training and Mints have the same effect as maxing your IVs and altering your Nature, but it's not actually the same as changing your Pokémon's IVs or Nature (in fact, if you check your Pokémon's Nature, it'll be unchanged).
This doesn't matter so much for battles, but when breeding Pokémon, max IVs obtained via Hyper Training and the effects of Mints will not pass down to offspring.
When to Use
We went through this previously, but it doesn't hurt to repeat this information again. Basically, you should only use Hyper Training and Mints as a last resort, because it usually takes a lot of time and effort to farm for the items/BP. However, if you're swimming in those resources, go crazy!
In general, you should use them for:
Legendary Pokémon, since you cannot breed them to improve their IVs. Nor can you use Synchronize to guarantee their Nature.
Gigantamax for the same reason as above. You can technically breed them, but the ability to Gigantamax does not pass down.
Shiny Pokémon for the same reason as above. Again, you can breed them, but the offspring will have the same chance of being shiny as any Pokémon (in other words, very small).
Pokémon you've bred for high IVs, but you can't get every relevant stat with max IVs. Or Pokémon with high IVs caught from Max Raid Battles, etc. Or you're lazy (that's OK!).
Pokémon you used during the main story, which you've grown attached to. Honestly, you're better off breeding new Pokémon, but if you can spare the resources, why not?