Is there a pool for HD in 5.5 or is it just an infinite resource? I see a ton of ways to use them, but no pool r restriction on that.am I missing something?

We have this definition in the Glossay:

Hit Point Dice

Hit Point Dice, or Hit Dice for short, help determine a player character’s Hit Point maximum, as explained in chapter 2. Most monsters also have Hit Dice. A creature can spend Hit Dice during a Short Rest to regain Hit Points. See also “Short Rest.”

And the number of Hit Dice is limited for your class and level. I.e. Wizards:

Hit Dice have been renamed Hit Point Dice in 2024.

In addition to the explanation given above from the Glossary, the rules for this mechanic are given in the Character Creation chapter:

Hit Point Dice. Your class’s description tells you the die type of your character’s Hit Point Dice (or Hit Dice for short); write this on your character sheet. At level 1, your character has 1 Hit Die. You can spend Hit Dice during a Short Rest to recover Hit Points. Your character sheet also includes space to note how many Hit Dice you’ve spent.

and

Gaining a Level

When you gain a level, follow these steps:

1. . . .
2. Adjust Hit Points and Hit Point Dice. Each time you gain a level, you gain an additional Hit Die. Roll that die, add your Constitution modifier to the roll, and add the total (minimum of 1) to your Hit Point maximum. Instead of rolling, you can use the fixed value shown in the Fixed Hit Points by Class table.

and

Add together the Hit Dice granted by all your classes to form your pool of Hit Dice. If these dice are the same die type, you can pool them together. For example, both the Fighter and the Paladin have a d10 Hit Die, so if you are a level 5 Fighter / level 5 Paladin, you have ten d10 Hit Dice. If your classes give you Hit Dice of different types, track them separately. If you are a level 5 Cleric / level 5 Paladin, for example, you have five d8 Hit Dice and five d10 Hit Dice.

Once acquired via the above methods, the mechanics for expending and regaining these Hit Point Dice are pretty much the same as they were in 2014 with one notable exception:

In 2014, when finishing a long rest, you were supposed to only regain half of your Hit Dice.  A ton of people were not playing that way because that's not obvious or intuitive and so they either did not even know about this rule, or they were intentionally ignoring it.  The new rule in 2024 allows a creature to regain all spent Hit Point Dice upon finishing a long rest, bringing the mechanic in line with how many people were already playing:

Short Rest:

Spend Hit Point Dice. You can spend one or more of your Hit Point Dice to regain Hit Points. For each Hit Point Die you spend in this way, roll the die and add your Constitution modifier to it. You regain Hit Points equal to the total (minimum of 1 Hit Point). You can decide to spend an additional Hit Point Die after each roll.

Long Rest:

Regain All HP. You regain all lost Hit Points and all spent Hit Point Dice. If your Hit Point maximum was reduced, it returns to normal.

Regain All HP. You regain all lost Hit Points and all spent Hit Point Dice. If your Hit Point maximum was reduced, it returns to normal.

Thanks, the long rest part information is really what I was lacking. I must of overlooked or not reached that part. Thank you.