grammar.se.indirect-object-substitute
Se #5: the fake se — le lo becomes se lo
Before lo/la/los/las, le and les surface as se: se lo expliqué. Pure phonetics — this se has nothing reflexive about it.
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Explication
Spanish doesn't allow two object pronouns starting with l in a row, so when the indirect le/les would meet a direct lo/la/los/las, le/les changes to se: le + lo → se lo, les + las → se las. So 'I explained it to him' is se lo expliqué, not *le lo expliqué.
This se is purely phonetic — a disguise for le/les — with nothing reflexive or impersonal about it. The order stays indirect-before-direct, and the pair attaches or moves as a unit: dáselo, quiero dárselo, se la mandé.
Because se here hides le or les (him, her, you-formal, them), it can be ambiguous; clarify the receiver with a él, a ella, a usted, a ellos when context needs it: se lo di a ella.
Exemples
The address? I'll send it to you/him/her now.
Région: global
I handed them over to the tenants.
Région: global
The invoice? I sent it to the accountant this morning.