For example OpenSSL library libssl-dev

Go to packages.ubuntu.com and look for download source package (link for bionic version of openssl)

There you will get some archives:

• debian.tar.xz - where all build scripts and patches are
• orig.tar.gz - package's source code

So now the question is: how to apply patches to ubuntu deb package manually? According to Ubuntu Packaging Guide you need to use a special tool: quilt

sudo pacman -S quilt
export QUILT_PATCHES=../openssl-patches
quilt applied # funny but no patches applied at this step
quilt push -a # apply all patches