The damage and health of mobs matches your hero’s level. So, if you’re taking a level 70 hero through the Campaign, then all mobs are level 70 (or higher for bosses). Also, the difficulty at which you play puts multipliers on their health and damage. This is covered in the game guide…
https://eu.diablo3.com/en/game/guide/gameplay/game-difficulty#general-changes
You either create a new hero for the season, or you can take an existing non-seasonal hero and use the Rebirth option on them. Rebirthing maintains the hero’s kill count and play-time, but turns them into a seasonal level 1 hero, and their gear is sent via the in-game email back to non-seasonal (where it must be collected within 30 days or it’s deleted).
At the end of the season, your seasonal heroes become non-seasonal. This article, whilst for the end of Season 14 (the last time they published one) describes exactly what happens at the end of a season…
https://us.diablo3.com/en/blog/22312196/season-14-ending-soon-8-31-2018
A lot of your questions would be addressed here…