Clarification how to use view_cell, layouts or views

Hi, I'm not sure of the "correct" way either - I do it like this:

[...]
<!-- I also may have a little, hidden form embeded in this file, that is located in app/Views/modules/popform.php, like so: -->
<?= $this->include('modules/popform') ?>
</main>

If someone more senior sees this and cringes please tell me before I write a lot of stuff this way. Or is it good?

Right, that one was missing. What is actually the difference between  $this->include() and echo view() ?