www.grammarly.com This is a paid subscription service used by 500, 000 people at universities. They now offer individual subscriptions for a reasonable rate. Very good plagiarism checker and other writing support functions. For free, the best one I have found is www.plagiarismchecker.com. I use both regularly.
I checked almost all the plagiarism tools before answer this question. I check a document with Turnitin, then with grammarly (https://preparmy.com/recommends/grammarly ) and then with prowriting Aid.
Turnitin is checked and show similarity reports from Internet sources, Publications, and Their own database as well. Number of text many other factor responsible for their higher similarity report.
Grammarly (https://preparmy.com/recommends/grammarly )is also showing Internet source, Publications as well. But their plagiarism percentage is lower because if we add one or two text between any sentence then grammarly not consider it as plagiarised.
ProwritinAid. It show lowest because it only checked through the Internet Source not all the source like publications and others.
Other tools like Plagscan
These all the tools are using Grammarly Ghost api to check the plagiarsim they are not good to use according to my perspective. They also have some limit on number of words to check.