Updated:

1. Instant full health

/effect give @s minecraft:instant_health 1 10 true

Instant Health fills your hearts at once. The final true hides particles. On Bedrock:

/effect @s instant_health 1 10 true

Avoid huge amplifiers such as 255; they may not heal correctly. 10 is plenty.

2. Refill hunger

/effect give @s minecraft:saturation 1 10 true
/effect @s saturation 1 10 true

The first line is Java, the second Bedrock.

3. Slow regeneration

/effect give @s minecraft:regeneration 30 1

4. Heal everyone

/effect give @a minecraft:instant_health 1 10 true
/effect give @a minecraft:saturation 1 10 true

5. Healing zone (Java)

Put this in a repeating, always active command block:

/effect give @a[distance=..5] minecraft:regeneration 2 2 true

Anyone within five blocks keeps healing.

6. Clear bad effects

/effect clear @s

On Bedrock use /effect @s clear. See the effect command guide.

7. Heal pets and undead mobs (Java)

/effect give @e[type=minecraft:wolf,distance=..10] minecraft:regeneration 10 2
/effect give @e[type=minecraft:zombie,distance=..10] minecraft:instant_damage 1 1

Regeneration heals nearby wolves. Undead mobs such as zombies and skeletons are reversed: Instant Damage heals them and Instant Health hurts them, which is handy for healing a boss.

Troubleshooting

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