I was a little surprised to see the name of a prominent streamer in the rpg/tabletop community mentioned multiple times in that filing (assuming it's the same person).
I listen to a few, mostly those with a comedic bend (Glass Cannon, Dungeons & Daddies, Crit Juice, etc), but I don't have the time or patience for 3+ hour podcasts. Even 2 hours is pushing it. If it's not roughly an hour or less, I'll typically pass.
The point where it appears/originates is the target. You don't necessarily need to see it, but as the targeting rules state, you must have a clear path to it, so it can't be behind total cover.
The rules don't explicitly call it "line of effect", but it's explained in the same terms. Transparency would only factor into the degree of obscurity (or lack thereof), not cover.
https://www.sageadvice.eu/2018/01/26/if-a-character-is-mounted-on-a-broom-of-flying-can-they-use-their-dash-action/
When you take the Dash action, you get extra movement equal to your speed. If you have more than one speed, the relevant speed is the one you're using to move. The Dash action...
Wow, I remember listening to a few episodes of this podcast shortly after it started and being incredibly impressed by their audio post production work. Awesome pull for them to get Goldblum. I might have to go back and pick up where I left off.
Just a friendly reminder/suggestion to check out the first issue of Arcadia, the D&D zine from MCDM (Matt Colville), for expanded rules on mounts.
https://shop.mcdmproductions.com/products/arcadia-issue1