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TL;DR: As the DM, I want to prepare my group of newbie friends for the best first experience I can give them. I'm concerned about halts to give explanations ruining the flow of the game and negatively affecting immersion in the story. I thought I might have them play short one-shots in various settings against different events, unrelated to the campaign, instead of letting them dive into it and tailoring the beginning around their inexperience.

This question might sound too broad, so please allow me to clarify.
I'm the only experienced player and my closest group of friends has asked me to DM a campaign for them, as they'd love to play D&D all together. However, they are concerned about not being able to follow the huge block of rules and have asked for guidance (to which I said "just read the spell description"... Alright, sorry). They are all completely new to D&D and have never played a RPG before. I'm concerned about them being unable to handle a situation and me being forced to halt the session to give explanations, disrupting the flow and pace of the game and negatively affecting the immersion in the setting. However, perhaps making mistakes through the main story and seeing how they could be relevant in the future might be more instructive.

I'm already intervening on the building of their characters, while also helping them interwine their backstories with the lore of the world and giving support with the definition of their role in the party and the main storyline. I don't want to Pontius-Pilate the issue of mechanics knowledge by saying "read the Handbook, then we'll start when you're all done", as the one they look up to have a fun first experience. They asked to be taught about how they are supposed to act in-game (e.g., "Rather than asking to roll on Investigation to look for a hidden door, tell me how Barendur feels this room isn't all there is and starts touching the wall on the right...").

They feel like they need some sort of preparation, a "warm-up" before diving into the main game, and so do I. Aside from the classical Session 0, I've thought about different "one-shot situations" unrelated to the storyline to put their characters into, to help them better understand what they are supposed to do, what my role is and how the events unfold, e.g. "you're walking through a forest at dusk guided by your Ranger, heading towards the nearest village you know of, to ask for shelter after a long journey...", "you're in a tavern, sitting at a table sipping a hot soup. Suddenly, the guards break into the building, yelling and pointing at what looks like, at first sight from your distant table, a harmless civilian...", and some combat situations.

Do you think this is a good approach? If yes, what should I focus on? If no, then should I just start with the main story, carefully adapting the beginning to the players' inexperience while correcting and adjusting things on the way?

P.S.
Perhaps I'm just worrying too much. I really want their first time playing D&D to be the funniest experience I can offer. If this is still too broad and generic, I'm very sorry, please feel free to point it out and I'll delete the question.

"Start with a one shot" is effective, even with experienced players.

I thought I might have them play short one-shots in various settings against different events, unrelated to the campaign, instead of letting them dive into it and tailoring the beginning around their inexperience.

1. 
   

   That's what our D&D 5e DM did for our group the first time we got together.

   
   
   

   Our group had played various editions from the Original game to 4e, and all points in between. When 5e came out, we all created characters with the basic rules, or the PHB, ahead of time. We got together and he had us run through two small combat encounters before we began the "You meet in a tavern, and here's the offer" session that kicked off the campaign.

   
   
   

   That approach cleared up a lot of the training wheels stuff for the new edition for most of us. As we were all new to the edition we still ran into "Uh, what, how does this work?" questions as more class features and spells were unlocked. This seems to be a challenge for groups of all experience levels in this edition (and feeds this SE a lot of questions).

   
   


2. 
   

   I did this with my current group as DM

   
   
   

   All of our current group have played this edition before, but since some of the players to the group were new, my first few sessions were a stand alone vignette that will leap frog into the main campaign grounded in the recently published Gosts of Saltmarsh hard cover adventure.

   
   

An observation about your immersion concerns

Immersion into the role and into the setting varies by person. It can also vary from table to table based on the play group's play style that emerges after a few sessions together. With a newly formed group, you have to discover through play how much immersion each player prefers before you can assess how well your style is catering to that.

For example: as a personal preference, I tend to get more into character with my PCs in this edition in my brother's campaign. A few of the other players are far less interested in that style, and keep it all "beer and pretzels" ... and we are all good with that as a group.

About a year ago some friends and I - all four of us new to Pen & Paper - started into a campaign prepared by a seasoned DM. His solution to our inexperience was to lay out the basic rules to us, help us create characters (which took a while) and then start into the adventure, letting is getting familiar with the rules along the road. E.g., no magic in the beginning, etc.

The DM also designed our initial encounter to take place on a small island - so we had not that many options where to go, what to do, whom to talk to. Later we left that island, after killing the bad guy/necromancer. Basically, he created a demo for us, and then expanded it when we liked the game.

Disclaimer: This was done in Rolemaster, which has a different ruleset than dnd. Still, the general approach should be transferable.