The reason we don't allow link-only answers is that the link could be invalidated at any point, with the bulk of the answer diseappearing along with it. We require all answers to stand on their own, not through outside links. So what's left when we remove the links from the answer?
Probably the most consistent, easiest way to get a lot of XP is building an XP farm. It's a contraption that provides a constant stream of what is source of XP, in a way that is easily and safely obtainable. There are countless designs of XP farms, varying in difficulty and efficiency.
This part is not helpful on its own, as it requires knowledge of what an XP farm is. If someone doesn't know what an XP farm is or how it works, this answer doesn't help them in any way.
Probably the easiest in general, but with very low efficiency, is a spawner-based XP farm built around a random zombie or skeleton spawner:
This doesn't explain how to build a farm, or even what it looks like, leaving all the information in the video.
The next step up is a general mob grinder. The design by Sethbling requires minimum of resources, but takes some more effort to light the caves around.
Again, no explanation of what the design looks like, it's all in the video.
Easier, though requiring more cheap resources is "the same except built in the sky". with the kill platform at around 130 blocks above ground (or preferably sea, to avoid fall deaths) level, spawning platforms respectively higher. DON'T build the "cross-shaped" grinder that depends on mobs wandering around. It's an easy way to get Youtube clicks, not XP or mob drops, the efficiency of it is abysmal.
What's a "kill platform"? What are "spwaning platforms"? What's a "cross-shaped grinder"? This requires knowledge found in links, not in the answer itself.
If you're comfortable with the Nether, you can build a blaze farm instead - it grants roughly comparable amount of XP but the danger building it is quite obvious.
No mention of what such a farm would look like, or how to make it work.
Next, probably the most practical, providing the best XP gain rate relative to effort - very efficient and easy, is EnderMINI by Gnembon, a classic - which requires access to the End though.
The only thing we know about this farm is it needs the End in some way. Not helpful without the video.
After that we're going into realm of mega-builds: The classic "sky donut" by Ilmango has a great advantage of not requiring player interaction, while providing plenty of XP
Once again, only the name is present in the answer, nothing about how the farm works or how to build it.
A farm with even greater rates is a guardian XP farm, requiring lots and lots of work to build though:
Let's not forget XP silos - XP "builds up" slowly, over days, in these, but can be "retrieved" in seconds:
I figure you know what I'm going to say about these two.
And I think it should be included, even though it's no longer possible in the game, but for historical reasons the most efficient, and most epic XP farm ever deserves a mention: the fully automatic ender dragon farm.
This is useful to nobody playing on the latest version.
So what do we learn from the answer once the links are dead? That we need something called an "XP farm", but no details of what it does or how to build one. Someone reading the answer would then have no choice but to search the internet again, having learned nothing but the name of what they're supposed to be looking for.
Answers need to be self-contained, and this answer only points someone in the direction of where the answer might be.
One more thing. When a question is asked on a basic, useful topic such as grinding experience, make sure there's not aleady a duplicate that you can point the OP to, like "What's the most efficient way to get experience?"