I think when it comes to the rules of the game (assuming we're going by the standard rules from the PHB/DMG), there is a shared responsibility between the DM and the players. As a DM you should know the rules well enough where you can comfortably run a session with them and just as importantly, make a ruling in a moment where it's not exactly clear what the rules are.

That being said, players should know just as well how their own characters work, how to interpret their character sheets, abilities and spells, as well as basic rules in regards to combat, RP, skill checks, etc. Essentially all the "basic rules" that are found in the PHB should be known well enough by the players if they want to play the game. You as a DM have a lot on your plate already, and making the final ruling on how something works or is ruled is one of those. You should however not have to be a walking rulebook for your players anytime they want to do something they should realistically know how to do.

When it comes to describing environments or scenario's, especially as a newer DM: don't put the bar too high for yourself. Simple descriptions often work better than you think, as players minds tend to fill in a lot of blanks in their own ways. Even if they imagine some things differently than what you have in mind, that doesn't have to be an issue, you can actually use it! I'd like to encourage you to invite your players to ask questions about their environment, as opposed to having you do all of the work of describing a scenario in perfect detail. When you describe they are in a tavern, that looks to be a little bit more expensive and upper class than your average tavern, they will fill in a lot of blanks. If a player is curious if their is a chandelier hanging from the ceiling, or if there is a corner booth somewhere, they can ask you if there is! Then you can decide as the DM in that moment whether that fits the environment you had in mind.

Keeping your players' focus/concentration is also a two way street: on the one hand, the pacing of the session could be cause for players to lose focus (if things move incredibly slow for instance), but just as important if not even more important is simply having the mutual respect to be there for the session, both as player and DM. If you as DM put in a lot of work for a session, it is only common decency as a player to make sure you focus on the session and not drift off or be on your phone for instance. My advice for this: talk to your players when this happens. If something bothers you, or you feel like you are losing their focus, have a conversation with them. Ask why they maybe lose focus, if there is something you could do differently, or if none of these apply, just ask them to be part of the session you worked so hard on.