I'm looking for an online electronics forum (web-based)
where I can post some questions.

Get a ham license. Then you can find people who actively
design and build things, like to talk about them and would
gladly help you.

Amateur radio has largely become a plug 'n play hobby. Hams still build
antennas, and sometimes high-power finals, but other than small projects and
accessories, they mostly buy things. Don't expect to walk into a shack and
find a home-brew receiver these days.

One of the purposes in licensing amateur radio operators was to build a base
of people with both theoretical and practical knowledge of electronics. It's
not clear that it still does this.