Levels and points
Experience comes as points, and levels are a running total of them. Going from level L to the next costs 2L + 7 points up to level 15, 5L − 38 from 16 to 30, and 9L − 158 from 31 on, so each level costs more than the one before. Added up, level 30 is 1,395 points from zero, and level 30 to 31 alone is 112. The panel shows the points between the two levels you put in, the points each level is from zero, and what the next level costs. Checked against Java Edition 26.3; Bedrock Edition uses the same numbers.
Why spending levels late is cheaper
The enchanting table and the anvil charge levels, not points, and levels near 30 hold far more points than levels near 5. Three levels taken from level 30 cost 306 points; the same three taken from level 3 cost 27. So it pays to enchant when you reach 30 and to do small anvil jobs while your level is low.
Sources
The table gives the points each source pays and how many of it you need, on average, for the levels you put in. A bottle o' enchanting gives 3 to 11, most often 7. Mining coal gives 0 to 2, redstone 1 to 5, lapis and nether quartz 2 to 5, diamond and emerald 3 to 7. Iron, copper and gold ore give nothing until the raw ore is smelted, at 0.7 for iron and copper and 1 for gold; a furnace pays fractions as a chance of one more point. Most hostile mobs give 5 and a blaze 10. Silk Touch, spawners and mob farms change these, so take the counts as a guide.
Death and Mending
When you die you drop 7 points for each level you had, but never more than 100, as orbs anyone nearby can pick up. Everything else is gone, unless the keepInventory game rule is on. Mending uses the points you pick up to repair the item: each point mends 2 durability, and the points it uses never reach your level. A diamond pickaxe from broken to new takes 781 points.
Questions
How many points is level 30?
1,395 points from zero. Level 30 to 31 costs another 112.
How many bottles o' enchanting to reach level 30?
About 200 from zero, since a bottle averages 7 points. From level 20 it is about 121 bottles, because levels 20 to 30 hold 845 of the 1,395 points.
Why did I lose so many levels when I died?
You drop at most 100 points, which is level 7 from zero. Everything above that is gone, so dying at level 30 loses about 1,295 points.
Is it the same in Bedrock Edition?
The level costs, the death drop and the Mending rate are the same in Bedrock Edition. Some sources pay a little differently there, so the counts in the table follow Java.
Does it save anything?
No. The levels are kept in the page's address so a copied link opens the same numbers, and nothing is sent anywhere.