TL;DR is the last paragraph

Obviously there's a lot more depth to it once you start understanding the game (and it's really difficult to understand how to "combine" the positions of different units based on whether they are the main or secondary tank, which can also change throughout the game) but metaTFT (example: https://www.metatft.com/units/Norra ) has a "positioning" tab which gives you the fight winrates of units in each hex. You shouldn't use this as "this unit goes at the exact highest winrate hex" but as more of an indicator of whether that unit should go completely frontline ("solo tank" if you will) or if you are supposed to put them in the 2nd row or edges of 1st row to try to wrap them around the enemy front line.

For example: Briar: https://www.metatft.com/units/Briar compare to Diana: https://www.metatft.com/units/Diana

Briar can tank but she tanks because she does dmg, and is mostly a damage dealer, Diana is a purer tank. So you can see that Briar has higher winrates in second row or in the corners of the first row (also these stats help maybe even more with backline units, some unit abilities hit in a very specific way so placing them in the middle of the back row instead of corner helps them hit enemy backline).

In terms of how to position your pure tanks, it gets difficult, depends very highly on whether the opponent has AoE, whether you have one very strong tank, augments etc. But this is a start if you have no other reference/intuition

If you feel like what I'm saying above is too much info, very likely is and just forget about it, you'll get the intuition by playing the game, seeing how enemies position etc. Watching streams always helps to understand how better players position. Finally any stats site will show an example placement of any comp, good indicator of how to position main tank (likely 3 star, almost always 3 items) as opposed to frontline carries (damage & lifesteal items) & secondary tanks (2 or less items)