How bootstrapped works tldr - Percentile based confidence intervals based on bootstrap re-sampling with replacement.
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MIT OCW 18.05 has this to say about the technique:
https://ocw.mit.edu/courses/mathematics/18-05-introduction-t...
The bootstrap percentile method is appealing due to its simplicity. However it depends on the bootstrap distribution of mean(x') based on a particular sample being a good approximation to the true distribution of mean(x). Rice says of the percentile method, “Although this direct equation of quantiles of the bootstrap sampling distribution with confidence limits may seem initially appealing, it’s rationale is somewhat obscure.”
In short, don’t use it.
Use the empirical bootstrap instead (we have explained both in the hopes that you won’t confuse the empirical bootstrap for the percentile bootstrap).
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Updated to reflect suggestions in comments below.