Needs to be noted that while Solid File Explorer is quite capable and feature-rich with extensive features, it's only free for a two-week trial period. It's only two dollars so worth the expense but the OP did request 'free' solutions.
At one time, ES File Explorer was a really good file manager app, highly touted as one of the best file managers. Then it was bought up, by a company based in China, and the app was still a good file manager but with a lot of data mining added in. So remove ES if you still have it on your phone, it's now a privacy weak point. But while all that ES drama was going on, since that time there's been a lot of really good alternatives being developed. A lot of them are 'pay-for' apps though. Some file manager apps are more basic, working only locally (limited to the internal storage and microSD card). Some, like ES, also include some really handy networking support. Since there was no mention of just which features about ES you consider to be important (user interface? SMB support? FTP support?) this makes it more of a problem for actual, relevant suggestions.