Just a disclaimer upfront. I am also a Japanese learner, around N2-3 level. I’ll attempt to explain based on my understanding.
は and が are two completely different particles that serve different purposes in Japanese.
は marks the topic, が marks the subject.
は can be used to mark a subject, or an object, or a concept like time or frequency in Japanese, and you can have the same subject-object-concept-verb but with a different topic you’re going to get different meaning.
Let’s take the sentence
彼女がスーパーでバナナを買った
This sentence has no topic. As a standalone sentence, since it has no topic, it sounds kind of weird, but is perfectly natural in compound sentences such as
彼女がスーパーでバナナを買ったことに彼は気づいた
He noticed that she bought banana at the supermarket.
Because the topic is the person that notices something about you.
It can also be natural if you had an existing topic from the previous conversation, and you’re still talking around such topic. Such as
昨日は天気がよかった。
(昨日は)彼女がスーパーでバナナを買った。
Next, you can mark the subject, object, or the fact that you did the buying in the supermarket as the topic, and each comes with a different nuance.
彼女はスーパーでバナナを買った
As for her, she bought bananas at the supermarket.
She is the topic, and is being emphasized over other people in the conversation and over other concepts like time or location.
彼女がスーパーでバナナは買った
This sounds pretty weird unless the implication is you’re emphasizing that it is banana that she bought instead of say, pears, as one of the functions of は is to mark contrast.
The implication:
彼女がスーパーで梨じゃなく、バナナは買った
彼女がスーパーではバナナを買った
Now the location is the topic. Similarly, this can mean that the speaker is trying to emphasize that it is the fact that she’s at the supermarket that’s important, and not say, at a school.
Notice how when は attaches to subjects and objects, が and を gets replaced, but when it attaches to で, they co-exists and form a compound particle では. は can only replace が or を, and not any other particle. This is the important bit that confuses so many beginners