What to run
Open URL Encoder. It stays in the browser.
How to read the result
Decoding once is usually enough.
If the result still contains %26 or similar, it was double-encoded.
Do not decode in a loop until it looks clean if the data itself had a percent.
What to open next
Open Base64 Encode and Decode if a parameter then looks like Base64. It stays in the browser.
Open Redirect Checker if the question is where the URL goes, not what it says. The check runs from Cloudflare.