How do 2D tilemaps relate to game logic?

Hi, there.

First of all, I wanna apologize for my lack of general understanding. I've made a few generic "first game on unity from a course" games, and just started working on my first real project. I think I'm alright with general coding, but I'm struggling to understand a few Unity things.

The game I wanna make is just a simple 2D Tower Defense. At first I wanted it to be hex based, but I'm giving up on the idea and settling for an isometric view. After some searching I came across tilemaps, and they totally seem to be the way to go for designing levels. But here comes the problem. Alright, I set everything up and do some drawings and such. What do I end up with exactly? The tilemap itself seems to be just an image? Like a background or something? I thought the idea was to have a game object for each tile, having tile prefabs and such. But that's not really what it does?

How do I actually build the game with the grid system in mind after completing the tilemap then? I wanted squares/hexes to be highlighted when I hover any tile, I wanted towers to snap to those tiles, and units to move following them. But this tilemap system only seems to be a cute way to create a map background. Am I supposed to write grid scripts and game logic that are completely unrelated to that tilemap, and then just have them lineup perfectly?

In fact, let's suppose I want character to move freely. How do I even go about setting collision, types of ground movement speed, and all that kind of thing? It would all make so much sense if I was using tile prefabs, but with tilemaps I have no idea what I'm supposed to do. Am I missing something here? This doesn't make any sense to me.

Thanks a lot in advance!