To start the command syntax is disturbing :

/attribute <target> <attribute> modifier add <uuid> <name> <value> (add|multiply|multiply_base)

 Which would mean you have to enter a UUID if we just want to add two hearts of life to an entity? this seems very complex to me for an operation that seems so simple.

And then a UUID of what type?

I tried to give myself just 2 more hearts compared to my present value with the command ;

/attribute @p minecraft:generic.max_health modifier add UUID

but the command just stop at the UUID and return Invalide UUID and I tried all the syntax possible: I tried UUID:[I;xxx,xxx,xxx,xxx] , {UUID:[I;xxx,xxx,xxx,xxx]} , {[I;xxx,xxx,xxx,xxx]}  , [I;xxx,xxx,xxx,xxx] , [xxx,xxx,xxx,xxx] ,  {xxx,xxx,xxx,xxx} ...

with my player UUID and then I tried summoning entities and giving items with custom UUIDs and custom attributes but it didn’t work either.

For me it is therefore a very strange behavior, it would be more logical to have a behavior like scoreborad with selectors @