The most common way of saying 'if'. more accurately Past tense + ら, as 〜なかったら etc. are also valid. It is most close to Conditional. It implies a sequence: something happens after something else happens. Often proceeded by もし.
When conditions are met, something happens
Not a hypothetical, because you are pretty sure it will actually happen.
会議が終わったら、電話しますね。 I will call you when the meeting is over.
金持になったら、城を買う！ If I ever get rich, I will buy a castle! (you are sure you will some day get rich)
もし雨が降らなかったら、行きましょう。 If it doesn't rain, let's go!
雨が降りやんだら、どこかに行くと決めた。 We have decided to go somewhere when it stops raining. (It'll have to stop raining at some point right?)
something happened after an action.
Just like a sequence! often unexpected, but not always. Not really an 'if', more a 'when', 'upon (...)' or even 'after (...)'.
ドアを開けたら、サンタさんが立っていました。 When I opened the door, Santa was standing there.
休んだら、元気になった。 When I rested, I got better.
本を読んであげたら、弟が喜びました。 After I read a book for him, my little brother is very happy.
家に帰ったら、誰もいなかった When i returned home, there was nobody there. (I was expecting someone to be there)
Natural consequence
Note that it is less certain than と.
風邪を引いたら、くしゃみが出でます。 If you catch a cold, you'll (most likely) sneeze.
人を殺したら、{死刑|しけい}になります If you kill a person, you'll (most likely) receive the death penalty.
Emphasising a topic
Most close to なら(ば), as it emphasises the 'if it is (...) we are talking about, (...)'. Here it loses all function of a conditional though, essentially being like は.
お父さんだったら部屋にいるよ。 If it is dad we are talking about, he is in the room.
https://imabi.org/the-conditionals/
Asking for Advice
very common expression 「～たら 良いですか？」, lit. if I were to do (...), would it be(come) good?.
何を着たらいいですか？ What should I wear?
〜ましたら is a little unnatural
It is gramatically correct, but not very often used.
読んだら ○ 読みましたら △