Troubleshooting Boolean Issues Blender 2.90.1
I'm relatively new to blender, only have been using it and following tutorials for a couple months. Typically, with enough research I can solve whatever issue or mistake I come across, but countless google searches and this one still eludes me.
Currently I am following Derek Elliot's phone product animation video where he begins modeling a phone, adds a couple bevels and a solidify modifier for the body, then duplicates it retaining the modifiers (excluding the 2nd bevel) to create the port and buttons on the outer edge of the phone before applying a boolean to those intersecting buttons so that they cut out shapes from the phone body.
When I initially followed this method, my phone became &quot;hollow&quot; with the 'eyedropped' shapes just becoming transparent when set to bounds. (or wire.) I dug through the comments for tips but found nothing, moved to google and forum searching where there were similar problems, but none that solved mine. (manifold problems etc.)
I began to just try some trial and error within blender and have discovered:

Default cube resized functions (mostly) properly with a boolean.
Duplicated phone mesh with phone modifiers (tutorial video) did not work regardless of placement.
Separately modeled port shape works only when intersecting the top face of the phone.
Modeling the phone at both life size scale and fictional large scale produced the same problems.

Issue seems to come from straying too far from a default mesh. Too many vertices?
I had a similar problem and am in a similar situation to you (in that I'm relatively new). I believe the solution ended up being checking where I shade smooth and manually changing the geometry so I didn't get any weird artifacts. If I remember correctly, the issue is with the two meshes having different geometry, so blender struggles to match them in a pleasing way.
Hope this helps